pix of Custom built guitar amps/cabs

[Marcos - Munky]

In my experience, there's a few things that matters.

When I was testing this one outside of the enclosure, the boards position matters. I mean, when the tda board was closer to the 555 smps, it was noisy. But when everything was mounted in the enclosure, the smps and tda boards ended up very close to each other - but with the enclosure as a ground plane between them - and I got no noise at all.

If you're building the smps, a good layout helps to low the noise level. I once built a alembic tube pre with a (bad layout) integrated smps, the B+ tracks were at wrong places too near the signal tracks, and I got some high frequency bleeding. After that bad experience, I drew a layout for just the smps and use it as a separate board. I've powered a few circuits (alembic preamp, jcm preamp, fender twin-like preamp, ef184 preamp, mesa boogie preamp, ecl84 amp, 6n16b submini tube amp) using it and got great results.

If you're using a already built smps, you have to give it a try. Some of them are not made for audio and can be noisy (I have a dc step up board that's way noisy), while others aren't noisy at all.

The "main" power supply matters too. A noisy power supply without some good filtering will put noise in the circuit. This noise will go thru dc converters and can get noisier if there's no filtering. That said, some power supplies we suppose it's noise may give good results. I've powered a mesa boogie tube preamp using a sony psp power supply with no noise at all .

Your idea is very doable. I'd say to give it a try. Use a tested layout for the preamp (if you have one) and, if you're building the smps, use a tested layout too. I can send you the one I use if you want. If you're using a dc converter board for the smps, you'll have to test it to check for noise levels. Play around the board positions (maybe add a ground shielding around the smps if the boards end up closer to each other and you get noise bleeding) and the B+ wire, and keep in mind it'll probably sounds noisier outside the enclosure than inside of it, and you're good.

Btw, does this tube mic use transformers or not? So metimes I think on building me a tube mic preamp, but the transformers keep the idea out of my building list.

[rankot]

Actually, it is so called Shroom mic preamp (by Fred Nachbaur), transformerles indeed, but I changed it to use EF86 and 12A*7/6N2P. I still didn't build it, but I can open a new topic about it if your're interested. Already did a PCB design, but it could be better if someone could take a look at it before I start soldering

[Marcos - Munky]

I did a little research on it and found the schematic. Looks interesting. The thing is it uses 2 pentodes, which I'm out of them rn and tubes are somewhat hard to get here, so I can't add it to my projects list (at least for now). But I can take a look at the layout if you want.

Btw, for 220V out of that smps, a small heatsink may be necessary.

[rankot]

Started new topic here: https://www.diystompboxes.com/smfforum/index.php?topic=123502.0

[stallik]

I'm curious. When building a custom valve amp, what is an acceptable life for the tubes? I'm talking catastrophic failure not 'going off a bit'
It's entirely reasonable that the harder they are made to work, the less time they will last. Tonight, the Mesa 12AX7 in my little homemade champ(ish) amp ended it's life with a truly remarkable cacophony and had to be replaced. Extrapolating from a data set of 1 for each valve, they are lasting about 12 months.
I'm not complaining about this, the amp sounds brilliant and I'm prepared to accept this short life, just wondering what peeps thought was acceptable

[vigilante397]

In my experience 12 months is an extremely short life for a good tube in regular use. I haven't changed the preamp tubes in my main amp in over 3 years and they still sound great.

[stallik]

Thanks Nathan. I probably worded my question badly. I last revalved my big amps 30 years ago and have had 1 failure since then so I've always thought tubes lasted almost for ever.
When building your own amp however, it's possible to push the valves to hell and back in order to achieve some kind of magic tone, knowing that this will be at the expense of valve life. Doing this would be commercially unacceptable but in the DIY world, we can do whatever we want. So I did.

[vigilante397]

When building your own amp however, it's possible to push the valves to hell and back in order to achieve some kind of magic tone, knowing that this will be at the expense of valve life.

That's true, this is absolutely possible. I built a tube hi-fi amp from a kit several years ago and it sounded excellent, but the tubes seemed to go out very fast. So I poked around and found out that the voltages, even the heater voltage, were outside of the spec for the tubes. If you love the sound of the amp as is and you think it's worth swapping a tube every year or so to keep that tone I obviously won't stop you, always trust your ears. That being said you may want to check voltages, especially your heater voltages, to see if you can lower them without affecting the tone of the amp.

This is just my opinion, but I think even on custom builds you should be working within the limits of the tube and your tubes should last a very long time. If you can't get the sound you like without frying your tube, you may want to try a different tube

[stallik]

A good opinion Nathan. I am actually within the spec of the tubes but only just and that's quite deliberate on this build. The amp sounds pretty amazing.
The 6L6 blew after a couple of weeks, it's replacement is still going strong. That may be down to a duff valve. The 12ax7 has lasted since I built the thing so I'm going to leave the amp as is and see how things pan out

[amptramp]

When building your own amp however, it's possible to push the valves to hell and back in order to achieve some kind of magic tone, knowing that this will be at the expense of valve life.

That's true, this is absolutely possible. I built a tube hi-fi amp from a kit several years ago and it sounded excellent, but the tubes seemed to go out very fast. So I poked around and found out that the voltages, even the heater voltage, were outside of the spec for the tubes. If you love the sound of the amp as is and you think it's worth swapping a tube every year or so to keep that tone I obviously won't stop you, always trust your ears. That being said you may want to check voltages, especially your heater voltages, to see if you can lower them without affecting the tone of the amp.

This is just my opinion, but I think even on custom builds you should be working within the limits of the tube and your tubes should last a very long time. If you can't get the sound you like without frying your tube, you may want to try a different tube

There has been a steady rise in power line voltage over the years from 110 to 115 to 117 to 120 VAC here and the voltage at my wall socket is now 121 VAC.  If you are using a power transformer from the earlier years (or one that was designed in earlier years), you might benefit from a Globar negative tempco resistor.  These were originally common in series string heater wiring for televisions and were often sized for 450 mA or 600 mA heater strings but were available in almost any value.  They offered a soft turn-on without the line drop of a fixed resistance and extended tube life.  There was a unit that you plugged into a wall outlet and plugged your amplifier into and it provided this soft turn-on for items that didn't have it already.

Since you have established your heater voltages are too high, you have a choice of things to do:

1. Add a resistor in series with the heater output.  This would be a very low value resistor.
2. Add a fixed resistor in series with the power transformer primary.  The voltage drop can be tailored to the design.
3. Add a globar resistor in series with the power transformer primary.  You have fewer choices but it adds a soft turn-on.
4. Add a transformer with a low output voltage and connect it to buck the incoming line voltage so you can drop the voltage with no power loss.
5. Add an EMI filter in series with the power input.  This provides a small voltage drop.
6. Add a fuse or circuit breaker in series with the line input.  This provides a small voltage drop.
7. Add back-to-back diodes in series with the power input.  This adds a fixed voltage drop.

All of these things provide a line drop.  The heater current when the unit is switched on may be about four times the normal current in a hot filament and should be limited to extend life.  Resistance in series with the power input will give some sag to the B+ voltages which may be good or bad depending on how you want the amplifier to sound.  I do not recommend operating tubes outside of their ratings - that's asking for early death with no real benefit.

[vigilante397]

Thanks for the tips Ron. I fixed my amp a couple years ago as soon as I found the problem, Valve Wizard's website has some tips (most of which you just covered) on lowering heater voltages, and that helped me fix it, so my tubes last a lot longer now.

[Marcos - Munky]

I'm curious. When building a custom valve amp, what is an acceptable life for the tubes? I'm talking catastrophic failure not 'going off a bit'
It's entirely reasonable that the harder they are made to work, the less time they will last. Tonight, the Mesa 12AX7 in my little homemade champ(ish) amp ended it's life with a truly remarkable cacophony and had to be replaced. Extrapolating from a data set of 1 for each valve, they are lasting about 12 months.
I'm not complaining about this, the amp sounds brilliant and I'm prepared to accept this short life, just wondering what peeps thought was acceptable

This is very personal, as said. If you push the tubes to a point of lots of stress (bias them too hot), the lifespan will be shortened, but they'll overdrive early and you'll get a different and warmer tone out of them. If you bias them too cold, the sound will be cleaner and brighter, and the lifespan of the tubes will be longer.

That said, it depends on how cheap/easy to get are the tubes for you, and from where your tone mainly come from. If you mainly get the tone from stompboxes, colder biased tubes could be better. But some people get their tone mainly out of the tube amp, so they bias the tubes a little hotter, at the expense of the lifespan. I've read somewhere that Brad Paisley's EL84s barely lasts for a month.

[stallik]

A month? That's nuts!

I'd decided to repeat the tests I'd done when I built the amp, just to confirm that plate dissipation was still as I thought. Went from the beginning checking voltages without valves fitted. Everything completely normal, reached for the 6L6, knocked it off the bench

While I'm waiting for replacements to arrive, I pressed my boogie back into service. This reminded me that at bedroom volumes, it's not a great amp whereas my little 4 watt sounds magnificent. At 12 o'clock it's about as loud as it will get. Any more just adds overdrive but I prefer to leave it below and add pedals. Difficult to describe but the sound is really full and fills the room. I play better through it.

I'll redo the measurements but if my valves last 12 months, that's good enough for me. After all, I'm 1/12 as good as Brad

[Marcos - Munky]

if my valves last 12 months, that's good enough for me. After all, I'm 1/12 as good as Brad

That's why I think my valves should last at least 5 years

[amptramp]

I have a Muzak 920B storecast amplifier that is rated for 15 watts or 20 watts if you bridge across a resistor that goes from the centre tap of the high-voltage winding to ground.  It uses 6L6GB outputs with cathode resistor biasing so these tubes are just loafing at 15 watts and hardly more than that at 20 watts.  This is an amplifier designed for use 12/7 in a store where it is expected to be on from 9 AM to 9 PM every day.  I have a Stromberg-Carlson PA amp that is rated for 30 watts and uses 6L6GB outputs as well.

Both of these units operate their tubes well within ratings and don't suffer from idiotic tube consumption.  You can still get a warm sound from low levels of bias but use lower levels of plate and screen voltage to avoid excessive power dissipation.  If you don't like the power at lower voltages, go with push-pull parallel.  Chances are, you will get the sound you want without the tube death rate you don't want.

If you don't want low power, you can get 75 watts out of a pair of 6146 transmitting tubes.  The Ampeg SVT uses six of them in push-pull parallel to get 225 watts in a handy 35 kg / 77 pound package.

[stallik]

Got Home early today to find my new tubes on the mat. Checked my voltages and everything's nominally where it was when I built it. Plate dissipation is 12 watts, plate current 63ma
Just noticed a 1 watt resistor which is becoming a little discoloured - working fine, amp sounding great but I'll pop in a 2 watt anyway.

[willienillie]

Just noticed a 1 watt resistor which is becoming a little discoloured

Power tube cathode resistor in a Champ?  Yeah Fender used 1W but it wasn't enough.  I just put 5W in those and forget about it.

[Ben N]

In general the ratings of the resistors on and around power tubes in Fenders was surprisingly low, especially considering how well made and reliable those amps are generally. They should probably be upgraded as a matter of course.

[stallik]

Just taken a punt on 6 electrics from eBay. Fender, Dean, Yamaha and PRS SE. All in distressed condition with stickers all over the place. Should keep me busy in case I'm quarantined in the coming weeks. See if I can get my £90 back but I'm hoping the acoustic Fender with a nice green quilted top could be made good enough to keep.

[Galego]