Low output volume GA-5 Les Paul build

[Jerry garrcia]

Did my first point to point build and things went along fine. Until start up
A full bridge rectifier since an old PT 250-250VAC. OT (from an old tape deck that I’ve used same model before in 5W SE builds before, with primaries 430 Ohm and secondaries 0.3 Ohm (exactly like the previous OT’s).
6SJ7 and a 6V6. 16, 10 and 10uF filter caps.
B+ 329, B+1 322 and B+2 245.
6SJ7
Vp 125 (Rp 47K)
Vs2 80 (Rs 240K, .47uF to ground.
Vk 2.35 (Rk 680R)

6V6
Vp 311
Vs 325 (screen resistor between pin 6->4)
Vk 19.9 which should be a plate dissipation of 89%. Rk 470R.
Shielded cables from input jack, to volume pot and from volume pot to 6V6.
First I had no volume at all but found out that the input jack had the tip and sleeve switched…

Now there is a total lack of volume. Like 5% of what should be expected.
Any suggestions on what to do to troubleshoot?

 

[Jerry garrcia]

Just realised that the screen cap to ground is missing….
Could that be the reason for the lack of volume?

[tubenit]

Just realised that the screen cap to ground is missing

Not sure what you're referring to?   What screen cap to ground?      Gibson_ga5.pdf (el34world.com)


With respect,  Tubenit

[shooter]

when the DC reading appear fine, AND you've eliminated the tubes by swapping or testing in another amp, it's AC signal tracing time.
scopes are real handy, a signal tracer is also real handy, OR
you can keep swapping parts til it's totally new again 

[sluckey]

Those resistor values don't match the schematic.

[kagliostro]

What screen cap to ground?

I think he mean the second filter cap of the PS (10ufd 450V)

Franco

[Williamblake]

He has a 6SJ7 that has a screen and that has more gain with a cap to the cathode or to ground.

[sluckey]

What screen cap to ground?

I think he mean the second filter cap of the PS (10ufd 450V)

Franco

I think he means the screen cap on the 6SJ7 preamp tube.

[PRR]

Just realised that the screen cap to ground is missing….
Could that be the reason for the lack of volume?

Yes.

[Jerry garrcia]

Just realised that the screen cap to ground is missing

Not sure what you're referring to?   What screen cap to ground?      Gibson_ga5.pdf (el34world.com)

 
With respect,  Tubenit

edit... fixed quote... sluckey

This one. It’s the Les Paul version.

[Jerry garrcia]

when the DC reading appear fine, AND you've eliminated the tubes by swapping or testing in another amp, it's AC signal tracing time.
scopes are real handy, a signal tracer is also real handy, OR
you can keep swapping parts til it's totally new again 

I’ve tried several sets of tubes and the same problem. Tested all resistors and caps before I placed them in their position.

Unfortunately no scope nor signal generator. Hate this. Was so happy that all the VDC’s were fine when I started it up since I did the PTP for the first time. Where should I start swapping?
Will try to swap the OT.
If one did have a scope, how should one do to trace it?

Yes.

I will install the 0.047uF and see. After some reading in the Merlin books I have some doubts that it will solve it all. What I understand (not a lot but some progress) it does increase the gain a bit. This “amp” is REALLY quiet. Like I have to sit in front of the amp to hear if it’s on.
When I’ve got you on the line I do wonder if you also could help me with an other amp? The reason that I ask is that I’ve seen that you answered a question before about a triode wired 6SK7. I have a triode strapped 6SK7 as a driver for a 6L6 in another amp. 6SJ7 up front, 5F2 tone stack, triode strapped 6SK7 and a 6L6. Great tone but not louder/punchier than a 6V6. I’m having a hard time understanding the characteristics of a triode  strapped remote cutoff pentode without any load lines nor tube data. Don’t think it produces enough Vp-p for the 6L6. Could change to a 6J5 or a 6SQ7 but not as fun. 

Those resistor values don't match the schematic.

True, but I’ve done some amps with the 6SJ7 and this bias sounds the best to me so far. So not true to the schematic. Also an 6SJ7 bias switch between grid leak and cathode bias. Checked it and seems to ground the right parts when switching. Will draw up a schematic tomorrow and post.

Not sure what you're referring to?   What screen cap to ground?      Gibson_ga5.pdf (el34world.com)


With respect,  Tubenit

It’s the 6SJ7 one. It’s all cramped up in there and I did (my first PTP) by just following the schematic and forgot to place the .047uF to ground.

[PRR]

> I have some doubts

Just do it. With huge un-bypassed G2 resistor the pentode gain may be less than 10. Even 5. With bypass, over 100. This is in line with your "5%" estimate.

[HotBluePlates]

Will try to swap the OT.

Yes.

I will install the 0.047uF and see. ... I have some doubts that it will solve it all. ...

You're willing to swap the output transformer (almost never a problem), but hesitate to install a missing capacitor (that will certainly slug the stage gain).

Reconsider that stance.  I think that (like me, and many others) you may be grasping at anything out of frustration because the Big Build didn't go as planned.  You'll get there!!

[Jerry garrcia]

Will try to swap the OT.

Yes.

I will install the 0.047uF and see. ... I have some doubts that it will solve it all. ...

You're willing to swap the output transformer (almost never a problem), but hesitate to install a missing capacitor (that will certainly slug the stage gain).

Reconsider that stance.  I think that (like me, and many others) you may be grasping at anything out of frustration because the Big Build didn't go as planned.  You'll get there!!

I will change the cap and hope for the best. The OT thing was before I realised the missing cap. True that the frustration took the over hand since everything else checked out fine (which was a big surprise  :laugh since I’ve never done PTP before and didn’t have anything to use as a template). When all the voltages were fine and grounding’s looked and tested out the OT was my culprit idea for malfunctioning parts of the circuit. But then it tested (DCR) like one that ran good.
If/when I get this to work as expected it might have to be moved to a smaller chassis or something else to be added. Too much space.
Maybe add another 6V6 to be run in parallel or a reverb (have an extra 12V tap from the PT and a 230->12V trafo so might be able to squeeze in a Lamington reverb..).
Long story short. I’ll add the cap and update

[Jerry garrcia]

And also. If one were considering on adding a tone control. Will a 5F2 tone stack drain to much gain so I will have to add another tube as a driver? If so, what would be a good octal to use?

[kagliostro]

No tone stack after an EF86 tube, preamp pentodes don't like Tone Controls, it is a heavvy load for them

You can try this (but it has only effect on Bass)



A better solution (exludin a Tube Cathode Follower) is to use a Souce Follower to perform the addition of a tone control of any type you can like more after an EF86



The Tone control here is .... don't remember what it is    (a ToneBone ???) but you can add also a one other one at your taste, as you can see there is a Morph control, you can use it also in your amp, the same for the Gain control at the cathode (use resistor and cap value compatible with your tube)

Franco

[Jerry garrcia]

So now I have added the screen cap to ground on the 6SJ7 (vacuum cleaning and doing our taxes can wait). Tried with three different tested sets of tubes.
Noticed a major increase in volume. Maybe up to five times compared to before. But unfortunately still a way to quiet amp. Still maybe 10-20% of what it should deliver. Did a sound test compared with a 5F1 I built. When the 5F1 is turned on by the switch in the volume pot the Champ is still a lot louder when on 1 compared to when the GA-5 is on 10. When the GA-5 is on 10 you can still have a normal conversation and not be disturbed.
Recorded and post a link.

Unfortunately it’s made by my phone so it has a volume clipping function.
Any tips on how to proceed with the troubleshooting is greatly appreciated. I don’t get it. Everything tests out fine with regards to DCR and Voltages. Filaments 6.6VAC.
Cheap and crappy jacks.
On the photo there is the blue unused 12V secondary.
Edit: Realised that I forgot to draw the ground lead on the bridge rectifier in the schematic. It’s in the “amp”.

No tone stack after an EF86 tube, preamp pentodes don't like Tone Controls, it is a heavvy load for them

Franco

Thank you! Will look in to that once this sucker is up and running.

[sluckey]

Your 6sn7 plate resistor is only 47K. That means low gain. Change to 220K per the schematic. Also change the screen resistor to 2.2M per the schematic. Any better.

Looking at your pics it looks easy to just completely remove the 47K. Unsolder the 240K from pin 6 and reattach to pin 8. Then put a new 2.2M on pin 6 to B+.

[Jerry garrcia]

Your 6sn7 plate resistor is only 47K. That means low gain. Change to 220K per the schematic. Also change the screen resistor to 2.2M per the schematic. Any better.

It will be an easy fix but filled with melancholy due to a total lack of good volume. According to the RCA charts the voltage gain will go from roughly 50 to 160. Might also decrease the voltage drop resistor before preamp B+ so I’ll get around 300VDC.

[JPK]

I also hear a significant hum with that low volume.

[Jerry garrcia]

I also hear a significant hum with that low volume.

Sitting next by with single coils and a LED over the chassis. That’s not the problem…

[Jerry garrcia]

So it is solved. Redid the Rp and Rs as original schematic but just a slight increase in volume. Then I measured one resistance that I forgot to check.
Changed one part and problem solved.

Guess which part…..

[Jerry garrcia]

 

Thanks for all help and support. Now it sounds really good and can’t really decide if I should go for the hotter settings with the Rp 220K and Rs 2.2M or Rp 47k and Rs 230K. Sounds a bit like two different amps. But very pleased that it’s hum free and sound good since it’s my first PTP.
Sorry for the total f**k up with the volume pot. Should have measured directly or jumped the coupling cap of the preamp to the grid of the 6V6.

So if anyone knows how to get more drive in to the 6SJ7-tone stack-triode strapped 6SK7-6L6 I think the amps will have to wait after Christmas when that’s solved. Sounds good but lacks drive. Sounds like a mellow SE 6V6

[shooter]

maybe just scrap the TS in the amp, since most modern guitars already control tone 
 

[sluckey]

EL34 is easier to drive than 6L6. Just connect pin 1 to pin 8 and use either tube.

[Jerry garrcia]

EL34 is easier to drive than 6L6. Just connect pin 1 to pin 8 and use either tube.

Already done that. Will give the EL34 a try. Also got a KT66 and a KT88…

[kagliostro]

Before to read this

So if anyone knows how to get more drive in to the 6SJ7-tone stack-triode strapped 6SK7-6L6

I was thinking the schematic of your amp was this



The addition of a Source Follower is still OK also now that I've read you have a third tube the Triode Strapped 6SK7 between V1 (6SJ7) and V2 (the 6V6)

if not, as other possible solution I was thinking to suggest you something like here (I prefer the Source Follower)



Now I can suggest to add the Tone Control after V2 (the triode Strapped 6SK7) but if you are lack of swing I think it will not be a good idea

---

About Humm, if you want to eliminate it I strongly suggest you to add a Gyrator to the PS

it wieghts a lot less than a real choke and also price of components is only a fraction of the price of a choke (not to mention a 155H one)



The Board is 50mm x 30mm

Till 100mA load the Mosfet didn't need an HeatSink, but better if you bolt one or to the chassis (note that if you don't use an insulated model TO220FP the IRF820 need an insulator between it and the HeatSink or Chassis)

Please revise Layout

Franco

[Jerry garrcia]

maybe just scrap the TS in the amp, since most modern guitars already control tone 
 

I know but have already drilled holes in the chassis and tube socket installed. Two channel? Adding a 6SQ7 channel 

[kagliostro]

The 6SQ7 has a gain of 100, in which way do you want to use it on the amp ?




https://www.youtube.com/watch?v=hulShQ0QcnY

Franco

[Jerry garrcia]

The 6SQ7 has a gain of 100, in which way do you want to use it on the amp ?

Franco

Could this napkin drawing work🤔? With an Ebb around 300V both tubes should have a gain about 50-60 with this setting according to the RCA data sheets.

[kagliostro]

Am I missing something ?

I don't see the triode connected 6SK7 tube so ...

Have I understand corlrectly ?

There Is a 6SK7 tube in your circuit ?

Franco

[Jerry garrcia]

Am I missing something ?

I don't see the triode connected 6SK7 tube so ...

Have I understand corlrectly ?

There Is a 6SK7 tube in your circuit ?

Franco

Not in this one. Sorry for being unclear. My fault. A napkin drawing while watching “Slow Horses” with the family. A great show by the way. Should have been a 6L6 output tube.
The first schematic included is the SE 6L6 amp with a lack of drive. The one on a napkin should have been labelled 6L6. Did exclude the tone stack and included a second channel, the 6SQ7 instead of that so the drilled chassis hole could be filled with a volume pot instead of the tone pot and a 6SQ7 instead of the triode strapped 6SK7. Friday and had too much to do this week. 

[kagliostro]

Don't worry about unwanted holes

an aluminium sheet can be a new faceplate with holes in proper number and position, problem solved

Now I think I've understand

Mr. Merlin is a strong supporter of pentodes in V2 position

Why don't arrange something like 6SQ7 > Tone Control > 6SJ7 > 6L6 (or 6v6)

think to the 5F2A only with a pentode in V1b position (V1a is the 6SQ7)



Better, this is the Selmer Zodiac Twin 30 with EF86 as V2



And this is the Matchless Clubman (V1 has triodes paralleled) with 6SH7 as V2



Franco

[Jerry garrcia]

Don't worry about unwanted holes

an aluminium sheet can be a new faceplate with holes in proper number and position, problem solved

Now I think I've understand

Mr. Merlin is a strong supporter of pentodes in V2 position

Why don't arrange something like 6SQ7 > Tone Control > 6SJ7 > 6L6 (or 6v6)

Franco

 
Will definitely give that a try!
Thank you!
My wife will not be as happy since she thought that  this “GA-5” would be the last amp before Christmas.
Might need a voltage divider since the gain will be quite a lot.
Always committed to a pentode up front. Don’t know why. To rearrange will be an easy fix. Unfortunately made on a turret board
So will take abit longer. Might as well change the PT since it is a bit to good for this amp. Just bought a couple of REALLY cheap toroidals from a Rumanian manufacturer for my next builds. Ampeg B15-NC, an amp for our HIFI (Rob Robinette Stereo amp) and a GA-30. Then maybe done🤔

[kagliostro]

Just bought a couple of REALLY cheap toroidals from a Rumanian manufacturer for my next builds. Ampeg B15-NC ......

If I can suggest

Schematics, Layouts and all that you can need on the links (two different clone versions of the amp of your interest)

http://ctgelectronics.weebly.com/custom-amplifier---fcbn-51b.html




http://ctgelectronics.weebly.com/custom-amplifier---fcn-51-b.html




Franco

[Jerry garrcia]

Just bought a couple of REALLY cheap toroidals from a Rumanian manufacturer for my next builds. Ampeg B15-NC ......

If I can suggest

Schematics, Layouts and all that you can need on the links (two different clone versions of the amp of your interest)

Franco

Thank you. The layouts are really useful. Especially since it’s supposed to be a really noisy amp. Will mod it some since I’m going for the one with the tone stack in between the two halves of the triode and probably go for a single channel and those dreadful shielded cables all around.