6G15/AB763 Build - and a problem/question

[pieterfx]

Hi!

I am a long time reader, and at this point I thought to start a thread as I need some help..

Over the past months I have build a 6g15 reverb circuit connected to a single channel AB763 circuit. Basically the original 6G15 into a Rob Robinette Blackvibe 6V6 (but with bigger power transformer).
You could also say basically this circuit https://el34world.com/Forum/index.php?topic=27175 but without the tremolo, and with 6V6's. And I put it all in a Princeton chassis, which barely fits. Luckily temps are okay, even when playing hard and long.

Some problem (and hummmm) chasing I did in the beginning was rewiring the inputs with insulated wire, and making better grounding (in the pictures is still the old). Also, adding a 220k screen resistor to the reverb return / v3 grid as I saw in the Hoffman 6g15 that isn't in the original. The amp is now impressively low on hum I feel.

Some pictures attached
All in all, very proud and the amp works and sounds good. Almost...

In the band setting (surf band as you might have guessed) I play this thing loud, maybe slightly over the edge of breakup. Volume around 40-45%. Sounds great. Reverb is drippy. Sounds are clean-ish dirty with compression. But.. if I play a big full chord it starts big and loud and then drops in volume, maybe 25%, before coming back to full. Not directly, but  Is that sag but just way too much? If yes how do I fix that? or can it be something else?

The other thing, that might be connected to the first, pushing the volume know over 60% creates a horrible distorted mess. Doesn't sound like it should.. help?

Hoping someone has an idea. I can post all voltages if needed, or share a sound clip when I get to play at high volume again.

[AlNewman]

So, you built this amp, with 6v6 and no tremolo?

[pieterfx]

In short, yes
Single input
Because 6v6 there are some more differences in the ab763 circuit where I followed the blackvibe6v6
And I used different power and output transformer and gz34 rectifier tube.

I dont have a schematic for it, I do have a - crude - DIYLC layout that I can share tonight if it helps.

I was thinking the power drop maybe is caused by too weak power section.
But the transformer I have is rated at 160mA on the HV, the GZ34 I believe can do 250mA. And the amp needs about 115mA when using Rob Robinettes power excel file. So that cant be it..?

[AlNewman]

All things being equal, since the 6v6 would have less negative grid voltage, it could be as simple as attenuating the signal into the 6v6 grids.  That could be achieved by lowering the value of the grid leaks?

But, all other things aren't equal.  The fact that you temporarily lose signal could be a bias issue, or an oscillation, etc.

It would definitely help to post your voltages at least, if not an updated schematic as well, and everybody would have a better idea what is happening.

[pieterfx]

Here is a picture of the layout, I never made a schematic but might have a go later.

Voltages:

Nodes:
A 431V
B 427V
C 376V
D 360V
E 295V
F 250V

V1
p1 129
p3 1,6
p6 124
p8 1,88

V2 (6K6)
P3 289
P4 295
P8 23

V3 (rev recovery)
P1 160
P3 1,3
P6 250
P7 114
P9 125

V4 (preamp 1 and 2)
P1 200
P3 2,4
P6 234
P8 1,9

V5 PI
P1 214
P2 55
P3 82
P6 212
P7 55
P8 82

Hope this helps!

[AlNewman]

what about voltages for the power tubes?

[tubenit]

Based on the volume drop and distorted tone when pushed hard, I would suspect oscillation problems with the power tubes.


I would try a different pair of 6V6's.


If that didn't help ..................   I would try disconnecting the NFB. While there is often a squeal if the OT wires are "backwards", sometimes there isn't one until the amp is cranked. I've had that happen on one of my amps.  When the NFB is removed, crank the amp  .............. if the problem goes away try reversing the OT wires.


with respect, Tubenit

[pieterfx]

Powertube voltages are at 427-428 for both plates and screens. Bias at -40V.

Attached an improvised schematic.

[AlNewman]

Seems to me you may be biased pretty cold.  Did you install the 1R resistors at the cathodes?  What are your voltages at pin 8?

[tubeswell]

In the band setting (surf band as you might have guessed) I play this thing loud, ... But.. if I play a big full chord it starts big and loud and then drops in volume, maybe 25%, before coming back to full. Not directly, but  Is that sag but ...? If yes how do I fix that? or can it be something else?.

Yes that's power supply sag. Try another (fresh) rectifier tube.

[passaloutre]

It’s less than a dollar to install a couple 1N4007s across the rectifier socket and test.

[pieterfx]

Seems to me you may be biased pretty cold.  Did you install the 1R resistors at the cathodes?  What are your voltages at pin 8?

I dont have the 1 ohm resistors, so the pin 8 is 0v.
I dont think I am biased cold, using - again, its so great - Rob Robinettes webpage and measuring voltage drop across the transformer I get about 22 mA for both 6v6's, I actually dropped it a bit to 20mA after re-measuring.

I have an attenuator coming today so I can tinker at home with high volume. things to try based on your tips:

- different rectifier tube
- different power tubes
- try without negative feedback
- try diode rectifier

will let you know how it goes.
thanks for now for all the tips!

[pieterfx]

So, I tried a bunch of stuff, but no solution yet, and some weird findings I dont understand..

- tried different powertubes > no difference
- removing the negative feedback > no difference

> what struck me when I removed the negative feedback was that the resistance between the tip of the output jack and the point where the 820Omh NFB used to connect to the 56Ohm trail resistor was still about 57 Ohm. So this is the resistance from the tip of the output, through the output transformer, to the chassis ground, up thought the NFB trail resistor? Is that not too low for an 8ohm secondary? So maybe the output transformer is busted, could that be it? Or do I have the outputs and output transformer wired wrong?

- I dont have another GZ34 lying aroung, I do have a 5U4GB that I put in. No sound. Some static in the output but no sound.
- then I took out the rectifier and put 2 more diodes in. No sound. Very weird. I have voltage everywhere, a bit higher than with the GZ34, but its all there.

> one thing that stood out for me was 375V on V4 (preamp 1), pin 1. That used to be about 200V. All other voltages were more in line. including the pin 6 of the same tube.

[Esquirefreak]

When voltages suddenly goes way up in a preamp stage it sounds like one or several tubes are not conducting.

/Max

[sluckey]

> what struck me when I removed the negative feedback was that the resistance between the tip of the output jack and the point where the 820Omh NFB used to connect to the 56Ohm trail resistor was still about 57 Ohm. So this is the resistance from the tip of the output, through the output transformer, to the chassis ground, up thought the NFB trail resistor? Is that not too low for an 8ohm secondary? So maybe the output transformer is busted, could that be it? Or do I have the outputs and output transformer wired wrong?

This is totally normal. The OT secondary is very low resistance. Doesn't indicate any problem.

> one thing that stood out for me was 375V on V4 (preamp 1), pin 1. That used to be about 200V. All other voltages were more in line. including the pin 6 of the same tube.

Maybe the cathode has lost it's ground connection. Check resistance from V4 pin 3 to ground. Should be 1.5K

[pieterfx]

the resistance pin4 to ground is 1,5K as it should.

But its not conducting, also no voltage when turned on. The other half of the tube is, also 1,5K and has about 1.8volgt when on.

Changed out the tube, no change!

It worked with the GZ34, not with the diodes now...

Nevermind, I got the tube conduction, after a bit of re-soldering.

So now I have it working with the diode rectification... Still does the same volume drop at very high volume levels. So its not power section/sag related? It feels / seems to me the amount of signal is just too much for the power tubes or phase inverter, so it kind of craps themselves.. is that possible? can that be because the 6g15 circuit gives a bigger signal?

[sluckey]

Preamp 1 (V4) has no dc path to ground for the grid. Without a dc path to ground the tube cannot establish bias. Correct this by adding a 1M between V4 pin 2 and ground.

[pieterfx]

Okay

So that (the missing 1M grid to ground) was probably something I should have / could have spotted myself... DOH

It fixed it!

GLORIOUS, THANK YOU

[Bieworm]

What changes did you do on grounding layout? Does it deviate from the posted layout a lot?

[pieterfx]

I had the pre amp ground bus connected to the chassis seperate from the input ground. I changed that to a single shared ground point.
Also, I beefed up the ground bus wire which I feel helped also.