Question about transparency in ab763 style amps

[espinete]

I had a headstrong king reverb (heyboer transformers) and a 1966 super reverb (mercury magnetics transformers) with all carbon resistors, i believe. In both of them i disconnected normal channel, tremolo and NFB wire. I loved them because its clear sound and gain. Both had tung sol 5881 and a zener circuit to dispel extra voltage. Now i have a boutique amp with same specs, except the trannies. Hand made. Power tranny special made for these tung sol 5881 reissue tubes. The gain has an ugly crunch "'fuzzy" kind of sound when i combine the amp with any od pedal. And the transparency it's too much clear. I really don't like it. That's why i put a normal 820 ohms metal film resistor. And i swapped plate resistors , i put metal film ones. The crunchy sound that i don't like maybe it's a little bit better for my tastes now with od pedals. I tried to fix this issue with the builder but we couldn't get the same sound with pedals than i used to get with both Fender super reverb and Headstrong king reverb amps. I was going to swap some caps and resistors to see what happens. But i really don't know if i'll fix the problem. Because i don't know where the issue is coming from. The main question i wanted to ask you is why my new amp is clearer sounding with a 820 ohms in NFB than both headstrong and fender amps with no resistor in there? I have more metal film resistors now . Orange drop 715p and TAD caps. Could be the transformers , zener system,..? I've been talking with a guy with the same amp as mine and he had the same problem from the begining. So, i'm not mad.
Please, i really need some advice because i'm getting really frustrated. The amp sounds stellar without pedals and amp treble pot at zero. I believe i can fix this , no matter how much it costs . But i don't know what i'm doing and there are no good techs in my area .
Thanks in advance

[brewdude]

Do you have schematic?
How about some photos?

[10thTx]

IF the amp has a passive effects loop, you can put an EQ pedal in the effects loop that may allow you to shape the tone to the "transparency" that you want.

with respect, Tubenit

[espinete]

No effects loop. The question was about the transparency but the main issue is the "invisible" ugly crunch sound that appears when you connect any od pedal . It comes along with overdrive sound of the pedal and it sounds a little bit like a fuzz pedal.
Schematic is like an super reverb ab763 layout :
https://goo.gl/images/cPLiYr

I'm going to upload some pics

[espinete]

17 Pics here :
http://rubencebrianmusic.com/?page_id=110&preview=true

Just tell me if you want an specific area and do some more.

Orange drop 715p and Tad caps mostly. Xicon caps on power tubes. Carbon resistors everywhere except nfb 820 ohms metal film cap and vibrato channel plate resistors, metal film too.

Tone Stack mid cap in vibrato channel is a 0.047uF like Twin Reverb. Normal channel bright switch works like connect/disconnect normal channel. Vibrato bright switch works like two different levels of punch. The OT is multi tap. There is also a switch gz34/solid state and a fan

[espinete]

485 voltage
39,40 bias
I don't understand this eather. Tung Sol 5881's Maximum plate voltage is 400v

[Stevem]

You need to look and see which gain stage the feedback was connected to as compared to the first two amps you posted that you liked, also in the amp you do not like how many gain stages are there before the phase inverter and also does the bad sounding amp have a cathode follower stage in the preamp section?

Higher plate voltages in the preamp section of even only 20 volts can make a big difference in top end response / gain and harshness of tone!
Also orange drop caps in all thru the amp in general add a harshness to a amp that I can't stand even for pristeen clean tones!

[sonicmojo]

The one thing in your first post that really jumps out to me is you say the amp sounds stellar with the Treble on “0”. This is not my general experience with AB763 circuits and the treble control generally shapes the tone more than the other controls. I’d recheck some of the values and wiring in the tone stack.

[espinete]

also in the amp you do not like how many gain stages are there before the phase inverter and also does the bad sounding amp have a cathode follower stage in

No idea. I can take more pics if necessary. Is not possible to see what you asked with any of the pics i uploaded ? Sorry, i have not enough knowledges. I can do some new pics of the circuit taking the reference with the pots or tubes. For example, you can tell me i take a pic of the circuit or the rear side of pots, from v4 to v6 , etc.,.

[espinete]

I know how to bias and that's all. I should call a tech guy and tell him everyting you suggest .
Thanks very much. But, firstable , do you think i should install a zener diode circuit( or find a professional to do it) to drop the voltage to 400v ?
The amp is modded as i told you.
Also it has a 5751 in v2. I say this because maybe the measurements are not the same than original design. Same circuit but i want to keep the tung sol 5881 tubes (400v), normal channel and tremolo disconnected, 5751 in v2, ot multitap, middle 0.047uF vibrato channel tone stack cap, gz34/ss switch , etc.,..
Thanks a lot

[brewdude]

Which rectifier are you using?

[espinete]

Generally gz34. I have a switch gz34/solid state

[espinete]

I'd like trying drop 85v with back-biasing circuit. Could you please tell me if this is a back-biasing circuit and if so, is it correct ?

[sluckey]

I'd like trying drop 85v with back-biasing circuit. Could you please tell me if this is a back-biasing circuit and if so, is it correct ?
I think : 4.7k resistor , 50uF cap and zener diode 1N4007 of 1A.
And i'll give my technician your instructions

It's a 470Ω resistor and a standard rectifier diode, not a zener. That is your bias circuit. Don't attempt to put a back biasing circuit in this amp!

[10thTx]

drop the voltage to 400v

Higher plate voltages in the preamp section of even only 20 volts can make a big difference in top end response / gain and harshness of tone!

I know of one Super Reverb amp that has a power transformer that is 354-0-354.

A useful general guide I use is this:

solid state rectifiction 1.4 x 354 = 495 volts

GZ34 1.3 x 354 = 460 volts

5V4 (or 5U4GB) 1.2 x 354 = 424 volts

So, IF you are wanting to lower voltage, then you can consider a 5V4 rectifier if you are not drawing more current then the 5V4 can handle. OR use a 5U4GB for more current handling. See attached "tube data cheat sheet".

Having said all that, I don't think changing the voltage will change anything about the tone stack issue needing treble on zero. I would be suspicious of a wiring mistake for the tone stack or some value that is off in the tone stack.

With respect, Tubenit