Valvestate 8080 with quiet Boost channel

[Stratfordade]

Hi Guys, I have the same issue of the first topic in my 8080: low volume in boost channel.
Voltages at tubes are pretty the same.

1 - 181V
2 - 110V
3 - 110V
4 - 5,8V
5 - 5,7V
6 - 110V
7 - 0
8 - 0,675V
9 - 0

D3 Cathode - 266V

I replaced IC1 and IC4 but nothing is changed. IC3 and IC5 are working because the switch is works correctly. I don't have replacement at home so I cannot try to replace them.

I see signal loss after led section and after contour section but I think is the design of this amp. Tube section seems work correctly it amplify the signal a lot!

Any suggestion?

I’ve just been working on another 8080 that had lots of issues with the preamp section including low volume and non-working reverb, and fixed it by replacing 3 or 4 of the TL072 and one TL071 (other op-amps hard to find but this did the trick).

Sounds like a shotgun approach but different sections of the op-amps used in different parts of circuit, I was seeing lots of odd results between input and output of op-amps, and they are cheap and fast to change. They do seem prone to failure after 30+ years. I socket any I change.

[tommymariot]

I tried everything. I resoldered all pads. Changed IC1 and IC4 (tl02 and tl071). I'm looking how to test ic3 and ic5 (M5201).
I used the oscilloscope and checked the signal amplitude:



1 - Ic1 pin 1 18v pp

2 - Ic1 pin 7 8v pp

3 - Ic2 pin 6 gain max 28v

4 - Ic3 pin 5 gain max od1 28v

5 - R34/r68 16vpp

6 - R68/pin 7 4vpp

7 - R71/r36/pin 3 22v pp

8 - C34/treble 22v pp

9 - Ic4 pin 3 4,5v pp

10 - Ic4 pin 6 4,8v pp

11 - C14/r16/vol pot 1,6vpp

12 - Ic5 pin 2 10vpp

13 - Ic5 pin 7 1,6vpp

14 - Ic5 pin 5 clean 20vpp

15 - Ic5 pin 5 od 3vpp

here you can find pictures of the scope.

https://www.dropbox.com/scl/fo/afxum...7bs7v7wxm&dl=0

It seems I loose signal between Tonestack and volume pot.

[g1]

I don't understand the big signal voltage drop across R68. Is that jack connection from there ok? It is supposed to mute the signal at the tube when nothing plugged in. If there was a problem there it would not affect the clean channel.

[tommymariot]

Yes confirm, is the jack mute section when nothing is connected to the amp. Old owner soldered a cable because he replaced the jack and broke the mute track.

[g1]

You are losing 75% of your signal across R68, why?

[Helmholtz]

You are losing 75% of your signal across R68, why?

Maybe because the NFB makes the grid a virtual ground point?

[g1]

Maybe because the NFB makes the grid a virtual ground point?

Thanks. I missed that pathway.

I think the signal voltage readings should be re-done with a much smaller input signal. 18Vp is way too big at IC1 pin 1. We end up getting signal levels limited by the LV supply rails.
Start with an input signal something like 50mV.

[tommymariot]

Normally I use 1khz 1Vpp. I will try reducing the signal later. I will post signal voltages. If I remove the tube I will loose signal anyway?