Help on new #40 build

[andresound]

Trying to provide as much info as possible. Just a thought. The pic below show how the relay system is grounded. This is the ONLY ground. Dave said to ground system in ONE place.

[martin manning]

My apologies... You do need a cap at the entrance to the loop driver stage because its grid is not at ground potential. You even asked about that above, and it didn't click! That may be why there is (almost) no output in OD. I edited the diagram above to show it.

I think you may also need a fixed divider to attenuate the signal at the output of the clean section in clean mode, but that can be addressed later.

The relay supply ground is fine as you have it.

[andresound]

Thank you, I will re-install the cap. Now we need to find out why I get nothing going into the return jack (using power section) . I think you told me how to jumper across the FX loop. Will check pevious posts.

[andresound]

Have this eagerly awaiting the "birth" of the amp. TL806 loaded with vintage EVM 12L

[andresound]

My apologies... You do need a cap at the entrance to the loop driver stage because its grid is not at ground potential. You even asked about that above, and it didn't click! That may be why there is (almost) no output in OD. I edited the diagram above to show it.

I think you may also need a fixed divider to attenuate the signal at the output of the clean section in clean mode, but that can be addressed later.

The relay supply ground is fine as you have it.

Just got home from work, re-installed the .05 cap and OD IS WORKING through the send jack. Far louder than clean!!! Shouldn't we be attenuating the output of OD instead? Anyway, we can deal with that later.

Dead quite through the power section. I was sure I had a little bit of level before the re introduction of this cap? Something up with the recovery/output stage.

One major problem solved . Now to tackle the next major problem. OUTPUT

[andresound]

Martin,

Thank you so much for your help in sorting this out!! It is guys like you that keep this forum alive. Giving selflessly and patiently of your time.
Thanks to Dave (colossal) and David ROOT for all your help too.

I am sure I have used up enough of the forum's resources for now . I will report back when I have the power & recovery section sorted.

Thank you to all

[martin manning]

Well, people with questions and problems to solve keep the forum alive as much as the people who are answering them!

For adjusting the OD vs clean volume you have a resistive divider at the OD output, which is where the Ratio control is on the later amps. You can reduce the value of the lower resistor. There is also a divider at the OD input, which is where later amps have a trimmer, but I would leave that as-is for now.

My concern is with the level of the input into the loop send stage, which must be kept from clipping. A good solution would be to move the control to the input, in front of the 0.05u cap, and put a 220k to ground at the send jack after the output cap. I would increase to pot to 1M, but otherwise this is just swapping connections around and adding the 220k.

You can jumper from the send jack to the front panel master volume pot's input to see if the signal is being lost in the recover stage of the power amp. Your DC voltages look reasonable, so there must be an open or inadvertent ground in the signal path. I'd also put a 12AT7 in the PI spot.

[andresound]

Thanks Martin,

At least we are getting somewhere. I do however feel a I'm "out of my comfort zone" technically. Having said that, I am truly grateful for the knowledge you guys impart. I am learning a great deal!! I am now starting to get a take on how this circuit works.

Regarding the control at the input to the OD. (do I install a trimpot infront of the .05? If this IS the case, I have a few spare. What value is needed?

Which pot do I increase to 1M?

Will do the bypass of the recovery stage when I get home. I cant imagine I got the output stage wiring wrong. The recovery stage circuit is VERY cramped and there could very well be a short to ground. I specifically went over that area carefully. Maybe I need new glasses!!!

Maybe new pre amp tube sockets are needed, (current ones are 50 years old!)?. I did change the power tube sockets out of necessity.

[andresound]

Martin,

I do have a 12AT7 in PI. Have a choice 4 NOS tubes. RCA, BRIMAR, MULLARD and TELEFUNKEN (NIB)

[martin manning]

I'll post a drawing later to explain my comments above.

[andresound]

What can I say.

Your are a scholar and a gentleman

[andresound]

Managed to poke around a bit more. Did as suggested by Martin. Bypassed the FX loop and output section NOW SINGS!!! Happy as pig shit. Played amp for an hour. Now to tackle the FX loop.

Noticed in OD mod (still to be adjusted), that if I turn the the OD level pot to minimum (off), there is no sound. Is this normal?

Tone stack behaviour seems strange, or I am just not used to it. If I turn the treble pot down, besides decreasing high end, it seems to give a BIG increase in volume. Is this normal?? Will revisit that section. A little confusing to wire switches upside down.

Thanks guys. I can now see the light at the faaaarrrr end of the tunnel A LOT of work still to be done.

BTW, A bonus is the amp is quiet, (no hum or exessive noise). VERY SUPPRISED and chuffed. LOVE this hobby!!

[martin manning]

Great! That narrows it down to the recover stage.

Yes, you can kill all the volume anywhere there is a control pot with it's CCW lug grounded. Turning down the treble means more mids, so I think more perceived volume is expected behavior.

Here's an updated schematic showing where the OD entrance trimmer and ratio controls are on later amps. If you want to add a trimmer, a 430k resistor and a 50k trimmer would replace what's there.

On the lower-right is my proposed mod to the send buffer.

[Smokebreak]

Martin,this is interesting, as I'm working on a loop, and am referencing various d'lator approaches.
So essentially you're replacing the 250K recover output(often used as global MV) with a 1M pot for the MV, and placing the normal MV(that's usually before the loop) "inside" of the loop, or at the beginning of it, as the drive. I'm referencing it like that, as my loop would be switchable in/out.
I guess it kinda boils down the where you put the pots(front or back), and how you label them;)
Is this your preferred way to run this loop, with a 1M after it, instead of 250K?
I hope that made sense, I've nearly confused myself.

[andresound]

Great! That narrows it down to the recover stage.

Yes, you can kill all the volume anywhere there is a control pot with it's CCW lug grounded. Turning down the treble means more mids, so I think more perceived volume is expected behavior.

Here's an updated schematic showing where the OD entrance trimmer and ratio controls are on later amps. If you want to add a trimmer, a 430k resistor and a 50k trimmer would replace what's there.

On the lower-right is my proposed mod to the send buffer.

Martin Thanks for that