I've been modifying an old Dynavox 6AQ5 amp for guitar and thought it might be a good idea to have someone else review my schematics.
I'm waiting for parts to the tone stack. The amp sounds pretty good without it, and seems to have enough volume. It distorts fairly early (around 3:00), but adding the tone stack will change that.
I used a simple concertina phase inverter without feedback in order to increase gain. It was an experiment that worked great.
There is an empty spot for a second preamp tube, but I decided not to use it in favor of the simpler design.
Thanks!!
Schematics
http://www.naturdoctor.com/Downloads/Dynavox_1.pdf
I'm waiting for parts to the tone stack. The amp sounds pretty good without it, and seems to have enough volume. It distorts fairly early (around 3:00), but adding the tone stack will change that.
I used a simple concertina phase inverter without feedback in order to increase gain. It was an experiment that worked great.
There is an empty spot for a second preamp tube, but I decided not to use it in favor of the simpler design.
Thanks!!
Schematics
http://www.naturdoctor.com/Downloads/Dynavox_1.pdf
As drawn, you have unequal drive to the output tubes - it's close to being single-ended! The plate and cathode load resistors must be equal for equal drive levels (47K or so?). This will require two additional changes: the grid of the concertina will be biased positive, so it will need a coupling cap, and gain will be greatly reduced (to a gain of ONE), so you'll need the extra preamp stage. If you have a seven pin socket, use a 6AV6 or 6AQ6 (half of a 12AX7), or even a 6AU6. If it's nine pin, you have more choices.
Also - R24 CAN'T be 22K - typo?
Also - R24 CAN'T be 22K - typo?
Thanks Tom ... I'm glad I posted!
Yes, R24 is most likely not 22K. I haven't measured the resistors in the power supply.
It looks like I'll have to order some more parts for the extra tube/gain stage.
Is there a benefit for switching from cathodyny to to a long-tailed phase invertor?
I know that you get gain with the latter, but is there a big difference in tone/distortion?
Yes, R24 is most likely not 22K. I haven't measured the resistors in the power supply.
It looks like I'll have to order some more parts for the extra tube/gain stage.
Is there a benefit for switching from cathodyny to to a long-tailed phase invertor?
I know that you get gain with the latter, but is there a big difference in tone/distortion?
Updated amp needs checking
I've updated the amp by adding another 12AX7 and built it, but it doesn't sound right. It sounds OK with very low volume.
Either there's a mistake somewhere, or some of the caps I had on hand were the wrong values.
I'm concerned about the coupling caps (C1=.02, C5=.01) and the phase invertor cap (C6=.1, which should be .01). My guess is that C6 is off.
I've had it for the evening, and am going to play guitar.
Any suggestions?
Thanks again!
I've updated the amp by adding another 12AX7 and built it, but it doesn't sound right. It sounds OK with very low volume.
Either there's a mistake somewhere, or some of the caps I had on hand were the wrong values.
I'm concerned about the coupling caps (C1=.02, C5=.01) and the phase invertor cap (C6=.1, which should be .01). My guess is that C6 is off.
I've had it for the evening, and am going to play guitar.
Any suggestions?
Thanks again!
Would you believe that it sounded better with C5 wired incorrectly?
Oh well.
Here are some voltages
V1.a 6=158, 8=1
V1.b 1=129, 3=1
V2: 1=173, 2=18, 3=51; 6=174, 7=18, 8=51
V3: 5=261, 6=237, 2=15
V4: 5=260, 6=238, 2=15
PS: A=287, B=265, C=237
The amp squeals with the volume off zero, so I may need to add NFB to reduce overall gain. I'm also driving the 6AQ5's slightly higher than the max (250).
Oh well.
Here are some voltages
V1.a 6=158, 8=1
V1.b 1=129, 3=1
V2: 1=173, 2=18, 3=51; 6=174, 7=18, 8=51
V3: 5=261, 6=237, 2=15
V4: 5=260, 6=238, 2=15
PS: A=287, B=265, C=237
The amp squeals with the volume off zero, so I may need to add NFB to reduce overall gain. I'm also driving the 6AQ5's slightly higher than the max (250).
triode is sweet till volume>3
I rewired the amp in triode mode and it now sounds very nice until the volume gets above 3, and then it squeals.
It's fairly low in volume at 3, and I'm wondering if that's as loud as it's supposed to get.
The voltages seem OK.
V1a: 6=159, 8=1
V1b: 1=131, 8=1
V2a: 1=164, 2=4, 3=13
V2b: 6=165, 7=4, 8=13
R10/11/12=4, R13/R27=3
V3: 5=248, 6=248, 2=15
I'm not sure what to do next.
I rewired the amp in triode mode and it now sounds very nice until the volume gets above 3, and then it squeals.
It's fairly low in volume at 3, and I'm wondering if that's as loud as it's supposed to get.
The voltages seem OK.
V1a: 6=159, 8=1
V1b: 1=131, 8=1
V2a: 1=164, 2=4, 3=13
V2b: 6=165, 7=4, 8=13
R10/11/12=4, R13/R27=3
V3: 5=248, 6=248, 2=15
I'm not sure what to do next.
I added a 47K grid stopper after the volume pot. I'm not sure, but I think it's a tiny bit better.
It sounds great until you get to about 3 on the volume and then it squeals. It's silent without the guitar plugged in, until the volume is above 3. Also, the clean is nice, but at 2.5 the distortion is horrible. It has hints of that squeal.
I checked the 12AX7's on the tube tester and they are fine.
I did notice that the 6G3 has a 15K resistor in series with the 100K plate resistor on the 2nd gain stage. The phase invertor is fed from between the two. Is this a gain reduction? Perhaps I should have put it in.
The other differences are this has cathode bias, whereas the 6G3 has a grounded cathode. Also, the 6G3 doesn't have R16 and R17, which tie the power tube grids to ground.
And ... It's originally a Dynavox amp for two speakers, which are typically an 8-ohm and 4-ohm tweeter. I have a 16-ohm Marshall speaker connected to the pair of wires with the higher impedance (1.1 ohms vs 1.0 ohms). I could try an 8 ohm.
I feel like I'm real close, but there are so many variables.
It sounds great until you get to about 3 on the volume and then it squeals. It's silent without the guitar plugged in, until the volume is above 3. Also, the clean is nice, but at 2.5 the distortion is horrible. It has hints of that squeal.
I checked the 12AX7's on the tube tester and they are fine.
I did notice that the 6G3 has a 15K resistor in series with the 100K plate resistor on the 2nd gain stage. The phase invertor is fed from between the two. Is this a gain reduction? Perhaps I should have put it in.
The other differences are this has cathode bias, whereas the 6G3 has a grounded cathode. Also, the 6G3 doesn't have R16 and R17, which tie the power tube grids to ground.
And ... It's originally a Dynavox amp for two speakers, which are typically an 8-ohm and 4-ohm tweeter. I have a 16-ohm Marshall speaker connected to the pair of wires with the higher impedance (1.1 ohms vs 1.0 ohms). I could try an 8 ohm.
I feel like I'm real close, but there are so many variables.
Could this be bad/old electrolytics in the power supply?
Is there a way to test electrolytics other than replacing them?
(This is a cap can, which is fairly expensive and difficult to take out).
I have another used/old cap can in the parts bin, but I'm not so sure about replacing an old one with another old one.
Is there a way to test electrolytics other than replacing them?
(This is a cap can, which is fairly expensive and difficult to take out).
I have another used/old cap can in the parts bin, but I'm not so sure about replacing an old one with another old one.
Phase invertor needs work
I had a hunch that it was the phase invertor, so I replaced R15 = 6.8K (used in the 6G3) with a 47K (used in the Matchless Spitfire). I also replaced R16 (2K) with a ground, since it's not used in the Spitfire.
Now it sounds sweet until the volume is 5, then the distortion gets ugly, and around 7 the nasty squeal sets in.
I'm wondering if the PI is being overloaded and needs more work.
Also, the Spitfire has only one preamp gain stage (a parallel 12AX7), whereas my design has two preamp gain stages.
Is it possible that I have too much gain?
Removing the second gain stage would certainly clean up the tiny chassis.
I'd probably end up with a clean amp, but that might not be too bad.
Any ideas?
I had a hunch that it was the phase invertor, so I replaced R15 = 6.8K (used in the 6G3) with a 47K (used in the Matchless Spitfire). I also replaced R16 (2K) with a ground, since it's not used in the Spitfire.
Now it sounds sweet until the volume is 5, then the distortion gets ugly, and around 7 the nasty squeal sets in.
I'm wondering if the PI is being overloaded and needs more work.
Also, the Spitfire has only one preamp gain stage (a parallel 12AX7), whereas my design has two preamp gain stages.
Is it possible that I have too much gain?
Removing the second gain stage would certainly clean up the tiny chassis.
I'd probably end up with a clean amp, but that might not be too bad.
Any ideas?
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