Schematic analysis of the Marshall Valvestate 8008

[Roberto]

Thanks ReadyTeddy,

I lost your post, saw it just now.
Thanks also to all other forumers, you helped me alot to understand how the whole amp worked, and how to repair it.
Finally I kept the amp as is, and I find that sounds nice.

[Roberto]

I'm missing the presence control on this power amp, because preamps usually don't have control on that frequency range.
Where do you suggest to intervene to add that control? Around C13 and R9 where there's the highpass shelving filter at around 7200 Hz?

[Roberto]

Short anwer, you can't.
It's a very complex amplifier, very well designed, has been tweaked as much as is possible, anything you do will make it sound worse ... believe me.

Searching something else, I've seen on another forum that you said:

My standard mixed feedback which I have been using since 1972 adds 2 voltages in series, one a sample of the output voltage (10K/220 ohms for a roughly 40:1 gain) and the 4 ohms speaker in series with a 0.1 ohms resistor, also roughly 40:1 , which combine for a roughly 20:1 gain.

Why those values?

I had been building Fender clones up to that point, when I switched to SS sound *was* different.

Instead of magic, I started measuring, scoping, etc. , to find *why* and what could I do to get a closer sound.

One thing I instantly noticed was that the tube amp signal at the output jack doubled (+ 6 dB) when unplugging the 4 ohms speaker.
My NFB network was designed to do the same.

Maybe not a *big* step, as Phatt says, but definitely a step in the right direction.

And instead of adding external EQ (which also works, of course), it cost me less than 10 cents, how to beat that?

Specially because I make my own precision 0.1 ohms resistors: 4 cm of 0.50mm diameter Constantan wire.

Still use that, 41 years later.

Can this apply to this amp too?

[Roberto]

Let me rephrase it better: the following filters seems to be added just for sonic reasons, and not to avoid oscillations or issues at DC.
- C3 and R3 make a lowpass at 4.8 kHz to dampen some possible fuzzyness of the incoming sound;
- C13 and R9 make a highpass shelving at 7.2 kHz that seems more a "studio recording" presence frequency;

Those frequencies seems more a compromise of the sounds used at that time (overfiltered and over feedback tube power amps).
Would cause any issue to remove the lowpass at 4.8 kHz and to lower the presence circuit to a more "guitaristic" 300 Hz, so removing R3 and raisinf C13 to 220 nF?

On top of that, can R56 be safely substitued by a 1 kOhm linear pot plus a 220 Ohm in series, to vary the current feedback?

Thanks

[Roberto]

Hi all,

I've found another 8008, with black front panel, that doesn't work.
The amp has been repaired by someone and broke again two hours later, so the owner decided to trade it.
I've traded it for a small treble booster that I've never used.

This is an older version with the fuses on the power amp board, and I would like to know which fuse holders it used at that time, because the ones that are now in are useless (just to say, the leads are shorting, so the fuse will never blow).

Then I have to buy the high power ceramic resistors R53 R54 R59 R64 R65 R66 to the needed 7W power, because as you can see from the pictures, they are clearly burnt.
I already have some TIP142 and TIP147 and will buy two TIP29 too. I will check if they are insulated from the heatsink, because I'm afraid the bolts will ground the TIPs.

I would like to substitute the linear/valvestate switch (that disables or enables the current feedback of R60 and R63) with a pot to decide how much current feedback give to the amp. I've seen something similar on old amps. Can you please advise me if there's anything to worry about, or ratio between voltage and current feedback that is better not to exceed?
I remember reading that for instrument amplification often there's a 1:1 ratio between the amount of the two feedbacks.

Thanks in advance for your help.
Best Regards
Roberto

[Reader]

I have some initial observations and questions

- - - I would like to know which fuse holders it used at that time, because the ones that are now in are useless (just to say, the leads are shorting, so the fuse will never blow).

I'm not sure what you mean. Can you explain the "short" you see. Your first photo shows a blown fuse in circuit board mounted fuse holder prongs but the fuse holder clips look OK.

- - -Then I have to buy the high power ceramic resistors R53 R54 R59 R64 R65 R66 to the needed 7W power, because as you can see from the pictures, they are clearly burnt - - -

They do not look burnt to me. The color looks like many resistors of that type look when they are new. You could measure the values and know they are good as is if the measured value is within tolerance.

[Roberto]

Please take a better look to those new photos:

Close to the FS1 label you can see the short of the leads of the fuse holders.
On the other photo, R53 and R54 look clearly burnt.

[Reader]

- - - Close to the FS1 label you can see the short of the leads of the fuse holders. - - -

I see a lot of the past bad workmanship. I'm thinking that you may be able to clean that up and then continue to use the existing fuse holders.

- - -On the other photo, R53 and R54 look clearly burnt.

OK. I see. You are talking about the helical black marks on R53 and R54 that look like they may be caused by overheating wire inside the resistor body. Correct?
The other resistors you originally mentioned seem to look OK to me. This is just my thought and they would not be a big deal to replace if that's what you decide to do.

[Roberto]

I see a lot of the past bad workmanship. I'm thinking that you may be able to clean that up and then continue to use the existing fuse holders.

I don't like to have wrong fuse holders there, that's why I would like to know which were the original ones. For sure I'll eliminate all the silicone that has been put there. What a mess.

OK. I see. You are talking about the helical black marks on R53 and R54 that look like they may be caused by overheating wire inside the resistor body. Correct? The other resistors you originally mentioned seem to look OK to me. This is just my thought and they would not be a big deal to replace if that's what you decide to do.

R66 has the same issue, that's why I want to come back to original ones over there too, like these ones: https://www.mouser.it/datasheet/2/427/cp-109226.pdf

Any suggestion on using a 0,47 Ohm 10W current sensing resistor with 100Ohm pot in parallel to sense the current for the feedback?

[Roberto]

Something like this one would fit, but I would like to know which ones were the original.
https://www.mouser.it/datasheet/2/358/typ_FAC-58104.pdf

[g1]

Ok, I think I know what is going on. We are looking at FS1 by the AC receptacle and seeing a regular clip in fuse.
You are talking about FS1 on the amp board, and I think those are the round solder in type (subminiature radial) like this:


I think someone has replaced them with something like this: https://www.belfuse.com/product-deta...n-0697h-series

I'm not sure where to get the radial style, maybe someone else will know. It is not critical. The pico type or the rectangular type are fine for replacement.

edit: try this one if it's 6.3A Fast you need: https://www.mouser.ca/ProductDetail/...CwutdF2w%3d%3d

[Roberto]

Thanks g1, very informative.
Any suggetsion on the variable CFB pot?