Troubleshooting Submini Tube Preamp

[Kevin Mitchell]

Here's something I've got on breadboard.


What's going on?
Without the 2nd tube stage - having the output on the center lug of the gain pot the sound is amazing. I really dig it. Clean most of the way through and some pleasant distortion when turned all the way up. But when that signal is fed into the 2nd tube stage the circuit starts to go to crap. In that case - when gain is cranked there's some nasty screeching and no guitar signal. When the gained is lowered the guitar comes in but with some nasty clipping. I've tried many variations of anode and cathode resistors as well as messing with grid stoppers on the 2nd tube stage. Nothing seems to clear up the noise that is made when the two stages are together as my schematic shows. This seems to be a problem with a few different circuits I've tried including the Superfly amp.

From what I've been learning - the 2nd stage is suppose to bring back some of the volume loss due to the tone stack before it. Since there isn't much of a tone stack there maybe there's simply way too much gain between the two? I'm hoping someone here has an idea of what's going on. I can't seem to figure it out. It's not a bad tube - Swapping the grid connection of the 2nd tube stage with the guitar input it sound good, just about the same result as the 1st stage but once they're mixed the problem occurs. So I'd have to guess there's way too much gain and idk what to do about it. I'm trying to get a loud preamp with minimal distortion (for a clean/fender style tube amp).

Any advice/ideas would be wonderful.

[amptramp]

This is a high-µ dual triode with a µ of 75 ± 15 and it appears that you may be getting feedback/oscillation or you may be clipping.  These devices run at 3.3 mA on the plate and the recommended cathode bias resistor is 325 ohms.  The plate resistors appear to be a bit high for this type of tube - you would use these values for a 12AX7 but that tube runs at a lower plate current.  You have not mentioned what the plate voltage is (200 V is the max).  You could try deleting the cathode capacitors and / or reducing the plate resistors to reduce the gain and bring the current up to the normal levels.

[Kevin Mitchell]

Thank you for sharing! I was observing quite a few schematics when I made those value choices I wouldn't have considered using smaller plate resistors or removing caps. Even Russian submini amps are designed in this similar way. I'm still leaning and am unaware of limits and restrictions for electrical applications. Half the info on the data sheet is over my head. So now on breadboard 470k is 13k and 220k is 7k5 I've also omitted the cathode caps. Much better! The gain is clean most of the way through.

B+ is ~190v  from a 555 smps. Resistors are 1/4w. I'll put up a clip once I throw in an output transformer.

[mth5044]

Perhaps try the last full schematic values on this page, ~1/2 down the page. Same tube, similar operating parameters.

http://www.dmitrynizh.com/submini-vibro-reverb.htm

[amptramp]

Good to hear that it is sounding better.  The tube you are using takes a little more current but less voltage than a 12AX7 (and the values you originally used would be suitable for a 12AX7).  If you look at the plate characteristics linked in the post by mth5044:



you will see that higher currents give somewhat better linearity (the difference between variations in plate current with differences in grid voltage).  You have also proven that removing the cathode capacitors drops the gain enough to avoid feedback-related problems.  Tubes are actually quite forgiving about how they are used, so a lot of values work - you just have to figure out what works best for your application.  You want to keep the current under control by using cathode resistors to provide bias that depends on tube current.  7K5 may be a bit low for a plate resistor but to each his own.

[PRR]

> when gain is cranked there's some nasty screeching

Output is getting back to input.

You got gain like 40 each stage, over 1,000 with two stages.

Any signal pin/wire is an antenna.

If 1/1,000th of the output signal "transmits" back to the input, it "will" self-oscillate.

Actually you have to meet all the Nyquest Criteria. Brain-busters. However phase is favorable for oscillation, that's the main thing. And haywire breadboard invites Murphy's Law, so it WILL self-oscillate.

High gain in a small space is hard to tame. That's why sub-mini tubes often do not come out smaller overall. MHz amplifiers need about an inch of length for each 20dB gain. You got 60dB gain so you may need to spread outs and ins three inches apart. Audio doesn't transmit so well, but still 60dB on a mini-socket is tough; sub-mini worse.

Study Fender Champ layouts. First 2 stage of Champ is about what you got there. Copy the layout and wire routes.

[tubegeek]

Yeah, the symptoms sound like oscillation and/or feedback - maybe both.

Ins away from outs, short leads (yes those are contradictory recommendations!) and possibly some bandwidth reduction on the 2nd triode. (Bypass the plate resistor with 20 pF maybe?)

[Kevin Mitchell]

Thanks for the advice. I'll try it out when I can.

I've got a half dozen 6p30b output pentodes in. I'm very new with tubes and can't figure some things out. Here's a datasheet. I'm trying to use these pentodes in place of a 6v6 to experiment. Some 6p30b data sheets imply dual anodes which is a head-scratcher for me.

Does the top grid row go to the cathode as many 6p30b and 6v6 circuits are drawn to suggest? Also, pin 1 of these tubes seem to be clipped and seems to be drawn as the cathode on the datasheet. So I'm a little confused and am looking for guidance.


I'm trying to swap the dual triode for two pentodes on the output stage of a custom superfly. Any help?

Oh. I couldn't get signal to the speakers last week. I think my 555 smps reached it's B+ current limit with the configuration I had. I'm scaling things down to tinker and waiting for parts for the 1771 smps. I'll very likely get a power transformer once I know what specs are needed.

[LightSoundGeometry]

Thanks for the advice. I'll try it out when I can.

I've got a half dozen 6p30b output pentodes in. I'm very new with tubes and can't figure some things out. Here's a datasheet. I'm trying to use these pentodes in place of a 6v6 to experiment. Some 6p30b data sheets imply dual anodes which is a head-scratcher for me.

Does the top grid row go to the cathode as many 6p30b and 6v6 circuits are drawn to suggest? Also, pin 1 of these tubes seem to be clipped and seems to be drawn as the cathode on the datasheet. So I'm a little confused and am looking for guidance.


I'm trying to swap the dual triode for two pentodes on the output stage of a custom superfly. Any help?

Oh. I couldn't get signal to the speakers last week. I think my 555 smps reached it's B+ current limit with the configuration I had. I'm scaling things down to tinker and waiting for parts for the 1771 smps. I'll very likely get a power transformer once I know what specs are needed.

I have two fender reverb transformers to sell , I would do 30 shipped to usa for both

[amptramp]

The 6b30b pentode would normally have the suppressor (the top grid) connected to the cathode or to ground.  Since it is beside the cathode, a direct connection would be OK.  Have you confirmed whether the clipped pin is pin 1 (cathode) or pin 5 (no connection)?  The drawing seems to show a cut pin that is not used in the pin 5 position.

There is only one anode.  The spec sheet gives you two maximum voltages, 250V with the tube conducting current and 350V with it cut off via a large negative grid bias.

[Kevin Mitchell]

The drawing seems to show a cut pin that is not used in the pin 5 position.

Yeah you're right. I assumed the clipped pin was 1 and the heaters correspond the same either way. And the datasheet does show as you're describing. I was confused about "dual anodes" because one drawing I was observing was a 6p30b-r; the dual anode version  I didn't notice at the time.

I have two fender reverb transformers to sell , I would do 30 shipped to usa for both

I found a source for toroidal transformers for tube amps with ideal specs. The only thing is; most of them seem to have dual 6.3v leads which might be unnecessary. But does that really matter?
I would be concerned that the reverb transformers (I hope you mean power transformers) wouldn't being able to provide the tubes with enough juice. These seem to be some hungry russians.

Hopefully this weekend I'll have time to work on the project.

[LightSoundGeometry]

The drawing seems to show a cut pin that is not used in the pin 5 position.

Yeah you're right. I assumed the clipped pin was 1 and the heaters correspond the same either way. And the datasheet does show as you're describing. I was confused about "dual anodes" because one drawing I was observing was a 6p30b-r; the dual anode version  I didn't notice at the time.

I have two fender reverb transformers to sell , I would do 30 shipped to usa for both

I found a source for toroidal transformers for tube amps with ideal specs. The only thing is; most of them seem to have dual 6.3v leads which might be unnecessary. But does that really matter?
I would be concerned that the reverb transformers (I hope you mean power transformers) wouldn't being able to provide the tubes with enough juice. These seem to be some hungry russians.

Hopefully this weekend I'll have time to work on the project.

they are the ones frequency central designed the mini tubes around ..I am just done with the tubes, I like the FETS much better for whatever reasons..but yeah they might not be the right spec

I think your extra wires are nothing more than extra taps in the coil .usually it has only a center tap if I am not mistaken

[tubegeek]

Or it's quite likely that there are two separate 6.3V windings - in series they will give 12.6V and in parallel (be sure to get the polarity the same on both!) they will give a higher current 6.3V capability.

The Fender reverb transformer is a small single-ended OUTPUT transformer. Not a power transformer.