Hi everyone!
By an accident I just got my hands on some pretty old (most of them are from the 60s I think) tubes and I'm wondering what can I do with them. I've never worked with valves before, but I think that is the perfect time to start now that I have some of them at hand. So I wonder whether there's some guitar related use for those tubes - a preamp? power amp? overdrive pedal maybe? Or some other pedal? I have a few of each:
ECC81 (a lot of those) - dual triode, equivalent to 12AT7
ECC82 - dual triode, equivalent to 12AU7
ECC85 - dual triode, equivalent to 6AQ8
STR85 - it's a voltage stabilizer I suppose
QQE03 - twin beam tetrode
EL42 - output pentode
EL84 - power pentode, equivalent to 6BQ5
EF95 - pentode, equivalent to 6AK5
ECH81 - triode-heptode, equivalent to 6AJ8
5763 - beam tetrode
Does anyone have any suggestions what to do with these?
Quote from: balkanizeyou on June 30, 2016, 08:13:09 PM
I've never worked with valves before...
<snip>
...Does anyone have any suggestions what to do with these?
Find a circuit that's already built, and needs replacement tubes, and pop them in. Or sell them.
That's a glib answer, I don't mean to come off like a jerk - but for 99% of valve applications, we're talking about voltages that can make you very, very dead if you don't know what you're doing.
That disclaimer out of the way, there's only one way to learn what you are doing, and that's to do it. The ECC81s, ECC82s, and EL84s are in loads of guitar amps. I think that for the ECC82 specifically, you can find some low-voltage boost circuits here - just search for "12AU7". 12AX7/ECC83 are more popular, but you can build a complete amplifier just with 81/82 in the preamp and EL84 as the output. Either guitar or hifi amp.
I've never heard of the QQE03 before, but as a *twin* tetrode it
might (I'm not looking at the datasheet, and might not understand it adequately if I were) be usable as a single-tube push/pull output stage of a low-power guitar amp. Could be fun.
Preamp stages can use a 12AX7 as the main voltage amplifier, a 12AU7 as the cathode follower and a 12AT7 can do both.
The STR85 is a voltage regulator tube with a strike voltage of 115 and a regulated voltage of 85±2 volts for a current variation of 1 to 10 mA.
QQE03 is a transmitting tube for VHF and low UHF frequencies with relatively low power.
EL42 is a Rimlock base power amp that puts out about 2.5 watts single-ended.
EL84 is a good output tube with good sensitivity.
6AK5 is an RF pentode that should also be usable for audio.
ECH81 Since the heptode has two control grids, it could be the basis for a good tremolo using the triode as an oscillator.
5763 is a beam tetrode that can put out 4.15 watts into an 8500 ohm load using 175 ohm cathode bias.
A lot of us grew up working on tubes and high voltages and we are still here. Actually, unless the circuit is on when you are soldering it, there is no reason for tube circuitry to be any more dangerous than battery-powered circuitry, but burns from tubes are a bit more common.
You don't say whether they are used or NOS (new old stock). If they are new, and you decide you want to sell, there are companies that will buy them...I have sold many to Antique Electronic Supply in Arizona. 12ax7a is going for $16 each, 12at7 fetches $4.50, EL84 brings $15. If you're interested, email greg@tubesandmore.com
Some of them are NOS, some are used - I know I could probably make some money selling them, but I'd rather keep them and make something cool out of it - I got them from a friend whose grandfather worked on one of the first polish computers https://en.wikipedia.org/wiki/Odra_(computer)
Thanks for the replies! I appreciate the warning, I'm well aware of dangers I'm going to face when working with valves. I've built a few solid stade amps and know how to handle mains voltage, so that's something to start with at least. But I'll surely read much more about safety before even attempting to build anything.
That aside, thanks for the suggestions! At least now I know most of them have some musical use. It appears that I can build a preamp using ECC81 and ECC82 or EF95 (6AK5) with ECH81-based tremolo, then make a low-power power amp using EL42, EL84 or 5763. Sounds like a good place to start.
Now, the problem is, that I have literally zero knowledge about designing valve-stuff, so I would be greatly appreciative if someone could point me in a direction of already working circuits - does anybody know a nice sounding pre-amp that uses my valves (or equivalents)? What about a some really simple preamp+poweramp in one circuit, like fender champ?
Another thing that crossed my mind is building a hybrid amp with a tube preamp and SS poweramp (say, a LM3886 chip running off a +-35V rails). The most problematic part would be probably building a power supply, but I'm wondering whether this approach would work in this case - I guess +105V is a respectable voltage for the preamp tubes?
(http://www.tubecad.com/march2001/Power%20Supply%20for%20Tube-MOSFET%20Hybrid%20Amplifier.png)
The way I see it, there are so many circuits out there - you're not re-inventing the wheel. Look at a pretty basic cookie-cutter topology: something like a common-cathode preamp, long-tailed pair PI, push-pull (or single-ended, or parallel single-ended - have some fun!) output section. Just maybe consider using some of those oddball valves in place of the usual suspects. Do you know any old-school tube EEs who can go over the datasheets and let you know which could work in an existing circuit? They will have different pinouts, but that's part of the creative part. :icon_biggrin:
Unfortunately not, in fact this friend of mine who was the previous owner of those tubes gave them to me because I was the only person he knew that could possibly do something useful with them (even though I've never messed with tubes) :icon_biggrin: so I'm on my own, but hopefully with help of this forum something will come out of it.
I want to build something really simple first just to get started - I was thinking of a simple preamp-in-a-box build, here's something I found on the webz that looks simple enough for me to build and it uses only one valve:
(http://i198.photobucket.com/albums/aa160/birdy81260/Fender.gif)
It's also perfect because I have a couple of 12V secondary transformers laying around, so the only expensive parts I'd have to buy would be the filtering caps (but now that I've checked they cost like $1 each, so not really) and tube sockets (hey, there's only one tube in here)
Unfortunately I don't have the 12AX7/ECC83, will it work fine if I put there an ECC81 or ECC82? Maybe a slight resistor values change will be required?
I'm not going to mount them on the PCB so the pinout should not be too big of a problem.
Start reading here (http://www.valvewizard.co.uk/), then buy the book, it's fantastic and so easy to follow. Great for peeps who are noobs to toobs and lots of useful stuff even if you've already got some design experience :D
Preamp-in-a-box is a good starting point; you've got enough tubes to build a slightly modified version of one of the AX84 amps (http://www.ax84.com/classicprojects.html), too. Build the amp, read some data sheets and set up the 12AT7s the way you like them in the preamp section, and get your feet wet with a good build and some tinkering. AX84 is also a fantastic forum for tube guitar amp stuff.
Probably a bit ambitious to dive right in at the deep end and design your own amp from scratch using oddball tubes. Not outside the realm of possibility, but much more homework required.
I've already been to Valve Wizard site, but thanks, that's a great resource! I haven't bought the book yet but I'll surely do.
And thanks for the link to the AX84 amp, I think I'll make this project next in line after the one I mentioned above. One question - I don't have a rectifier tube, I guess I can use a solid-state rectifier instead without any problems right?
The book's fantastic!
And yes, a solid-state rectifier will slot straight in :)
Quote from: amptramp on June 30, 2016, 09:48:33 PMActually, unless the circuit is on when you are soldering it, there is no reason for tube circuitry to be any more dangerous than battery-powered circuitry,
There's a statement that is not a fact and which is very unsafe advice as well. Some tube circuits omit a bleeder resistor. In which case you might have many hundreds of volts left on the filter caps after turn-off, and I wager you'd easily be able to tell the difference between being bit by that and by 9V. If you survived the experiment.
Quote from: tubegeek on July 03, 2016, 10:54:27 AM
Quote from: amptramp on June 30, 2016, 09:48:33 PMActually, unless the circuit is on when you are soldering it, there is no reason for tube circuitry to be any more dangerous than battery-powered circuitry,
There's a statement that is not a fact and which is very unsafe advice as well. Some tube circuits omit a bleeder resistor. In which case you might have many hundreds of volts left on the filter caps after turn-off, and I wager you'd easily be able to tell the difference between being bit by that and by 9V. If you survived the experiment.
Quite true if you have large capacitors and limited circuit drain or if you are using directly-heated cathodes (and none of these tubes are). I am used to the cathodes remaining hot for a couple of seconds, enough to drain the voltage from a radio but you are right - it may not be enough for a preamp. In that case, add a bleeder resistor or ground the filter capacitors before you start work.
If anyone is reading this thread trying to figure out how to use EF95 tubes, here's the answer: http://www.diystompboxes.com/smfforum/index.php?topic=120117.0 (http://www.diystompboxes.com/smfforum/index.php?topic=120117.0)
For the 7806, I used a 7805 with 2 1N400x diodes in series with its ground pin for 6.4DC. This does mean the 7805 tab/can cannot be grounded at the heatsink. I'm sure the 7806 is fine, but I happen to have plenty of 7805 and diodes to use.
You can build a Hi Octane with 2 12AU7 and a EL84, although it asks for 2 12ax7 so you'll have less gain. Excellent sounding amp.
There's a preamp using a ECH83 here in the forum, mayoe you can use the ECH81 in the same circuit.
Quote from: patricks on July 01, 2016, 05:14:34 PM
Start reading here (http://www.valvewizard.co.uk/), then buy the book, it's fantastic and so easy to follow. Great for peeps who are noobs to toobs and lots of useful stuff even if you've already got some design experience :D
And Merlin, the books author does pop in here and post every once in awhile.
dave
I'd recommend picking up some cheap modern tubes to test out your circuits, then swap in the new ones once they're finished. It would really suck to destroy some 60 year old tubes because of a minor beginners mistake.
Quote from: amptramp on June 30, 2016, 09:48:33 PM
Preamp stages can use a 12AX7 as the main voltage amplifier, a 12AU7 as the cathode follower and a 12AT7 can do both.
The STR85 is a voltage regulator tube with a strike voltage of 115 and a regulated voltage of 85±2 volts for a current variation of 1 to 10 mA.
QQE03 is a transmitting tube for VHF and low UHF frequencies with relatively low power.
EL42 is a Rimlock base power amp that puts out about 2.5 watts single-ended.
EL84 is a good output tube with good sensitivity.
6AK5 is an RF pentode that should also be usable for audio.
ECH81 Since the heptode has two control grids, it could be the basis for a good tremolo using the triode as an oscillator.
5763 is a beam tetrode that can put out 4.15 watts into an 8500 ohm load using 175 ohm cathode bias.
A lot of us grew up working on tubes and high voltages and we are still here. Actually, unless the circuit is on when you are soldering it, there is no reason for tube circuitry to be any more dangerous than battery-powered circuitry, but burns from tubes are a bit more common.
I made a tremolo/boost pedal with the ECH84 (really similar to the ECH81): https://www.diystompboxes.com/smfforum/index.php?topic=114688.msg1066515 (https://www.diystompboxes.com/smfforum/index.php?topic=114688.msg1066515).
I'm currently waiting on PCBs to refurbish the pedal because it wasn't a really nice build and i'd like to implement couple of mods. Should be here next week.
The triode has a gain of 22 so it isn't easy to make a phase shift oscillator with it.
The 6AK5 is a sharp cut-off pentode so you could make a preamp with it.
The ECC85 has doesn't have a centre tapped heater but otherwise I think is similar to the ECC81 (?).
Another +1 for Merlin's info. Especially, read the grounding advice. Compared to a lot of SS projects, I find there is a higher risk of getting annoying levels of ground loop hum with tube projects. Getting the ground connection paths right solves it.
If you want to make a small amplifier and keep it cheap with "found" parts, it is possible to use mains supply transformers "backwards" for the output transformer. Google to find out how.
One of my favourite types of output transformer is the distribution transformer. It usually has an 8 ohm secondary and primaries rated by power level. The most common ones are for a 70.7 VRMS line so a 1 watt tap is 5000 ohms, a 2 watt tap is 2500 ohms, a 1/2 watt tap is 10,000 ohms etcetera. They may not have the gap needed for a single-ended output but I have never run into a problem using them.
As for power, get a couple of wall warts, say 6 VAC and use the 6 VAC for the heaters and connect the 6 VAC of the other wall to the 6VAC of the first and the primary will have power line voltage on it and you can use a bridge rectifier or a doubler to get the plate voltage you want.
I have a 100V distribution transformer I took from an old distribution amp. I believe it has 8 ohm on secondary, but I am not sure how to check primary and secondary impedances. How to know power capability - to measure core size, as for power transformers, or there is another way to do it :icon_question: Please help!
If you put an AC source on the secondary, then measure the voltage on the secondary and the voltage on the primary you can get the turns ratio:
So if its n:1
n = Vprimary / Vsecondary
Then to calculate the primary impedance,
Zp = n^2 * Zs
where Zs = secondary impedance = 8 ohms
Eg. 100V, 8 ohm:
Secondary test voltage 6Vrms in
Primary Measured 27.4Vrms
Turns ratio calc: n = 27.4 / 6 = 4.57
Primary impedance calc: 8 * n ^ 2 = 8 * 4.57^2 = 167 ohms
Power estimation calc: Pest = 100^2 / 167 = 60W
Caveats:
- the test voltage must not saturate the transformer
- it's best to test at say 400Hz with a series capacitor to prevent DC loading on the amplifier.
- if you use 50Hz/60Hz mains you need to test at a voltage somewhat lower than the rating.
If not IR drops in the transformer windings will cause large errors in the estimate for n.
When you get Pest you could compare it against the size of other transformer. However,
the size has to do with the low frequency response.
Another thing to check is to measure the primary and secondary winding resistances.
You can also drive the primary and measure the secondary.
(Very roughly: A 60W transformer will be approx 1.2kg, 30W approx 0.8kg.)
Great Rob, thanks! I'll put 200mV source on one side and measure another, just to be sure not to make few kV. :)
Ohm's law to the rescue. If you have a distribution voltage rating and the tap you are using has a power rating (usually marked on the transformer), you use the equation:
V * V/P = R
If you have a 100 volt transformer, the 1 watt tap is 10,000 ohms, the 2 watt tap is 5000 ohms, the 0.5 watt tap is 20,000 ohms etc.
The transformer has to be able to pass the highest power it is rated for and the rating is the highest power tap. The tube manual entry for an output tube will give you the impedance the tube wants to see, which may vary with voltage applied. Tubes may have ideal impedances which are not available with your transformer but fortunately, it is not that critical when you are building an amp to just get it to work.
I have one, but no markings at all. I measured it's transformation ratio using 12AC few months ago, and concluded it must be some kind of 100V audio transformer, since it was connected at output of that old amp box and transformation ratio wasn't high, but stupid me didn't mark transformer leads when I removed it, thinking it will never be of any use... Now I want to make my first tube amp, and it seems that I can use it :) It has 4 leads on P side and 4 on S side, and when I put 220V on primary, secondary gives 52V (but I am not sure which of those leads I used to measure). So according to Rob's formulas, it has Zp of 143 ohm with 8 ohm Zs, and 70W of power (and yes, it is heavy). I will see what other leads do.
This page has some ready worked out PT as OT info...
http://locofonic.alphalink.com.au/valves.htm#avenue
I think it's cool that you might get something decent enough going with a PT that you already happen to have.
Great resource, thanks!
QuoteIt has 4 leads on P side and 4 on S side, and when I put 220V on primary, secondary gives 52V (but I am not sure which of those leads I used to measure). So according to Rob's formulas, it has Zp of 143 ohm with 8 ohm Zs, and 70W of power (and yes, it is heavy). I will see what other leads do.
If you are sure it was a 100V transformer then all the primaries probably connect together and all the secondaries probably connect together. Do you know how to work out the "outsides" of tapped winding by measuring the voltages?
220V on the primary might be excessive if you tested at a low frequency.
The thing that is tricky is if there is multiple secondaries you don't know which one is 8 ohms. Like if I saw 4 secondaries I'd be thinking common, 4 ohms, 8 ohms, 16 ohms. If you drive a test voltage V16 into 16 ohms and common then should be able to measure voltages:
8 ohm tap to common V8 = V16 / sqrt(16/8) = V16 / 1.414
4 ohm tap to common V4 = V4 / sqrt(16/4) = V16 / 2
[Forgot to add: you get the same conclusion if it's 32, 16, 8 or 8, 4, 2.]
If you have the old chassis or you can find a pic of the old amp you might be able to see labels 4, 8 16 ohms.
The thing that's harder to work out is which winding is 100V. It's common that the outer windings are 100V, ie the ones that produce the highest primary output with a test voltage on the secondary.
Just to be clear the power estimation formula assumes the following:
- we know the transformer is 100V
- we know what the 100V winding is
- we know impedance rating on the secondary testing winding.
If all these are not known the power has to be estimated another way. You can make an estimate from the size, weight, and winding resistances. You can also do winding heat rise test (which can take several hours to heat up) but you have to assume the manufacturer has used the lowest temperature rating wire.
Heat is almost never a problem with audio transformers. Saturation, bass distortion, usually sets a lower limit than heat.
In this case we know one side can stand 100V of "audio" (bass limit not specified; and sometimes quite poor) and the other side has parasitic resistance which should be "small" compared to speaker inpedance. 0.5 to 1.0 Ohms DCR may be appropriate for an 8 Ohm connection. (Cost is usually more important than economy in that business.)
Quoteparasitic resistance which should be "small" compared to speaker inpedance. 0.5 to 1.0 Ohms DCR may be appropriate for an 8 Ohm connection
For generic Tx's 0.8dB and 90% efficiency seems to be a good ball-park for transformers < 100W.
That implies the total secondary referred transformer DC resistance is,
Rt' = 0.1 * Rs (ie. Rs = 10* Rt')
Where you can measure, Rt' as,
Rt' = RDC_secondary + RDC_primary (1/n)^2
transformer n:1
It's a fairly loose estimate. Transformers which have good bass, or are good quality, or are on the large end may very well have lower Rt'. It only lets you estimate Rs not the power.
I suppose the bigger issues in using a 100V for a tube amp is the impedance match to the tube isn't going to be great and the core might saturate in a single ended stage due to the DC. There's definitely not going to be any air-gap on the core of a 100V transformer.
> It only lets you estimate Rs not the power.
If you know the primary voltage (100V), the turns ratio (not yet discovered), and a rough estimate of a happy secondary load, then you have a rough estimate of Power.
QuoteIf you know the primary voltage (100V), the turns ratio (not yet discovered), and a rough estimate of a happy secondary load, then you have a rough estimate of Power.
It works fine for single winding primaries and secondaries.
It goes a bit haywire for multiple primaries and secondaries.
With multiple primaries and secondaries:
For the secondary side it works best for the highest impedance tap. Do all measurements and calculations with the highest impedance secondary tap. It usually works out better that way. The lower impedance windings have lower DC resistance but it doesn't scale in proportion to the rated impedance because the mean-length turn varies (and the winding space in under utilized).
For the primary it's quite confusing to handle it correctly. All the primary windings are rated at 100V. The primary with the lowest number of turns gives you full power but the other windings deliberately drop the output power so you can "distribute power". The insertion loss and efficiency workout best based on the outer primary winding (highest number of turns). However a power calculation with 100V won't give anywhere near the right power with that winding. So for many transformers, you need to work out the secondary impedances using the calculations in my previous post. But you calculate the power using 100V on the winding with the least number of turns.
The above "normal" scenario is for generic transformers. The insertion loss for the full power case is worse than the low power case. Higher quality transformers might keep the insertion loss down for the higher power taps. The uncertainty in how the transformer is designed means you can have large errors in the estimates. One thing that saves the impedance estimate the true values are discrete values like 4, 8 etc. That means you can choose the whole number impedance closest to the calculated estimate.
A different scenario for the primary is if it just has 100V and 70V taps that's a whole different problem.
[Edit: fixed a few typos.]
Here's an example where the above method does work well if you use the primary outer windings to estimate the secondary impedance. This transformer is a better design in that the insertion loss is low (close to 0.8dB) for the highest power configuration (ie. the lowest primary tap)
https://docs-apac.rs-online.com/webdocs/0326/0900766b80326f99.pdf
If you estimate the secondary impedance using the outer primary winding you get 5 ohms instead of 16 ohms (a large error).
If you estimate the secondary impedance using the inner primary winding you get 15 ohms which is close to spot on with the expected 16 ohms; here the DC resistance for the 30W primary tap was estimated to be 27 ohms.
Be warned!
> Be warned!
Indeed.
I won't argue the typicality of that part. It is probably normal commercial grade.
Note that pins 13 to 18 are called "16 Ohms". And on the left we learn 13-18 reads 6.5mH.
6.5mH does not get up to 16 Ohms until 400Hz! Oh, it will couple an infinite generator to 16r just fine. But that generator working at 100Hz must supply 4X the reactive current as the real current that gets to the speaker. (And not saved by winding resistance: that doesn't equal reactance until 4Hz!)
Ah-- inductance measured with 1KHz. They should know better. An iron-core will show more inductance below a few hundred Hz (depending on lam thickness and resistivity). So it may be "OK" to 150Hz or lower.
Anyway: for a tube design the first choice is perhaps the highest impedance primary which is rated 10K impedance (1 Watt @ 100V). Or 2W which is 5K, or 5W which is 2. These may be decent matches for 25C5 class tubes at 150V of B+. While we should have a gapped core, my observation is that these things are so cheaply stacked that they are far from un-gapped. And since they turn up real cheap in odd places, we may forgive some bass error.
Quote6.5mH does not get up to 16 Ohms until 400Hz! Oh, it will couple an infinite generator to 16r just fine. But that generator working at 100Hz must supply 4X the reactive current as the real current that gets to the speaker. (And not saved by winding resistance: that doesn't equal reactance until 4Hz!)
That's a good point. Previously I did check that inductance predicted the turns ratio and it seems OK in that at 1kHz we haven't hit the lossy region: based on L, n=sqrt(Lp/Ls) = 24.2, and based on V, n=vp/vs = 25. So not bad agreement.
I didn't think about the bass response for the tube amp. Like you correctly point out it's not going to be that great driven by a tube. If you are lucky, those transformers give cutoffs specs for -1dB or -1.5dB or -3dB and usually end-up somewhere between 20Hz and 300Hz. But that assumes a *low source impedance* not a tube. With a low impedance drive on that transformer I get f(-3dB) = 7.9Hz and f(-1dB) = 30Hz for the 1W primary, and f(-3dB)=31Hz and f(-1dB) = 120Hz for the 30W primary. Those figures look quite acceptable under normal conditions but they don't apply when driven by a tube drive.
On the good side there are people on the web using 100V transformers as tube output transformers. So I guess you can get away with it to some degree. However, they seem to be push-pull designs which avoids the gaping issue but means the transformer has to have a centre-tapped primary.
http://www.ozvalveamps.org/optrans.htm
http://home.alphalink.com.au/~cambie/6AN8amp/M1115.htm
http://home.alphalink.com.au/~cambie/6AN8amp/Grant_Wills_6CM5amp.htm
For the single-ended case I suppose you could trade output power for core saturation by using the primary taps with less turns. You could also do something nasty like connect the tap to the +V rail drive the tube of one side of the winding then feed DC through the other side to cancel the field but then you might as well do a skewed push-pull. None of this is ideal.
I think I like the push-pull with the centre-tap.
My apologies for dragging out this thread but PRR's comment about the low frequency response inspired me to check how much error occurs in the turns ratio measurements when you test at 50Hz. What happends is the transformer inductance forms a voltage divider with the winding resistance and this makes the measured voltage lower and changes observed turns ratio. That's without considering saturation which will make things worse.
You get different results depending on which winding you apply the test voltage to.
For the example transformer I gave the PDF for:
Test voltage Observed Voltage Loss factor
Input on secondary (16ohm) 0.9952
Input on primary (1W) 0.9874
Input on primary (30W) 0.8302
So as expected the least error occurs when you drive the outer windings.
In this case driving the secondary and measuring the primary gives the least error.
On some transformers driving the primary and measuring the secondary will give a lower error as it depends on the relative winding resistances.