8008 Valvestate solid state amp with distorted output

[Georgio_P]

Back in the early 90's , before what we know as the Internet today really existed for most of us, Victor Litz used to mail out a paper sales ad.
I called in about a used Marshall 8008 Valvestate solid state amp listing for a few hundred dollars. It seemed like a fair price back then.
Salesman said that this amp would have no problem to drive my 4x12 cabinet with 75W Celestions wired in parallel to the full extent of their limits.
I paid around $20 for shipping. I was like 22 years old, short on cash, and was hoping this would fit the bill (hope is not a strategy).
When it arrived, I felt that it lacked the expected punch + gain @ 4 Ohms, hence barely powered this 300W cabinet, so I shelved it.
it never seemed to drive the 4 12's as expected, it did, but just barely...IMO..
No returns on used items, All sales final, and I was 6 states over, and they knew I was over a barrel.
Fast forward and it's 2024, I need a 2x12 combo amp, and held on to it all these years, thinking I could re-purpose this someday at a lesser capacity....
Reading some reviews from Reverb and other used gear sites, most say that the 8008 amp in general, should indeed produce more power than mine seemed to ever have.
Today, I just need this 8008 to drive 2 12" 16 Ohm 100W "KICS" speakers(another story) that I have laying around ( wired in parallel).
Expecting to get 8 Ohms on Channel B, only using Channel B for in / out connected to these two 12's...
Channel A to be unused.
I powered this amp up for first time in 10 years. It's been properly stored in a cool dry basement.
Red power light came on, great. It worked for two minutes with some humming (still sounded weak, but only producing 40W at 8 Ohms right?) ...then.....Pop and smoke!
Blew both channel fuses, but not the main. Channel A had no input or output.
I was running a Hotone Ampero into the channel B 1/4" input with substantial gain coming into channel B.
Now, I am waiting for new T6.3A 250V fuses to arrive, but what else should I check for in the meantime?
I am looking around for replacements for the two large blue bulged capacitors.
They seem to be SAMWHA CE-HC 85°C 50WV, 30mm x 30mm capacitors.
If I had to guess where they are, it would be at C11 and C14 in the PDF schematic, listed as 4700uF, and 50V.
Picture here for reference. The rest of the internals look OK to the naked eye (no burnt traces or components).
I have a DVM, but am unsure of what the issue is.
Many thanks in advance,
George

 

[Georgio_P]

I replaced the two big 4700uF caps, but no luck, Still blowing FS3 and FS2 with nothing plugged in. The power light only stays on for a second!
Should I start to look at the 6 output transistors?
No obvious burnt components still but on underneath of board it seems that the output transistors got hot at some point. The solder looked like it was oozing flux at the solder points, see pic.
Looking at output transistors next.
Thanks,
George

 

[mickeydg5]

Did you have Channel A control Gain at zero whenever you used just one channel and one cabinet connected to that channel?

I, myself, never ran an 8008 without using both channels with a cabinet on each.

If you replace those capacitors, go with at least 63V to 100V.

After that you need to evaluate and predict voltages in the circuits to figure out if anything or what else is acting up.

 

[Georgio_P]

Hello Mickey,
I did have Channel A at Zero gain when testing on Channel B connected at 8 Ohms.
I also tried both channels with no gain, no inputs, no outputs connected , and still blew at least one fuse.
Is there a good way to test the 6 output transistors in circuit with a DVM and touching the legs?
Most threads point to test them out as a next step when blowing fuses.
Also noticed that they are not easy to find online if needed, hence I need to know if they are shorted or not.
I replaced both caps with 50V, since that is what printed on the ones in there as OEM.
Thanks,
George

 

[g1.1]

Hi George. Not much action here for the solid-state stuff so I'll see if I can help.
For starters, the handbook says B input routes to both channels if used alone, so for your purposes you would want to use the A side of the 8008. Still, that should not have been a problem with no speaker connected to the A output.
FS2 and FS3 are not for the 2 channels, but for the + and - sides of the power supply. That means the output transistors might still be ok and it could be something like the rectifier bridge. Rectifier failure would be consistent with main capacitor failure.
You can check the output devices with your DVM set to diode check or low ohms range. Measure between collector and emitter, looking for a low reading or short (zero ohms). According to that datasheet, collector and emitter for the BDV's is pins 2 and 3.
As you noted, these transistors are obsolete, so there will be a lot of fakes out there. If you need to replace either pair, you can use TIP142 and TIP147. Get them from a legit source, like mouser or digikey.

 

[Georgio_P]

Hello G1.1,
Thank you so much. I will shift my focus to testing the rectifier bridge in that case.
A few hours later, I tested this onboard rectifier bridge. Put DVM in diode mode.....
I put my black probe on the (+) and touched red probe to other leads. I get readings between the (S) legs from the (+) and (S) to (-) legs, , but when going from (+) to (-), I get continuity, and a tone on my DVM.
So then I checked for reverse as well, so red was on (+) and black was on (-) and I get continuity, and a tone.
I just see some fluctuation on readings between (-) and (S), and between (S) and (+).
Does this continuity between (+) and (-) legs in diode mode mean that this rectifier bridge is shorted internally?
Mouser has these in stock for a few dollars, but the shipping there is 8X the cost....
Best Regards,
George

 

[yafal]

Back in the early 90's , before what we know as the Internet today really existed for most of us, Victor Litz used to mail out a paper sales ad.
I called in about a used Marshall 8008 Valvestate solid state amp listing for a few hundred dollars. It seemed like a fair price back then.
Salesman said that this amp would have no problem to drive my 4x12 cabinet with 75W Celestions wired in parallel to the full extent of their limits.
I paid around $20 for shipping. I was like 22 years old, short on cash, and was hoping this would fit the bill (hope is not a strategy).
When it arrived, I felt that it lacked the expected punch + gain @ 4 Ohms, hence barely powered this 300W cabinet, so I shelved it.
it never seemed to drive the 4 12's as expected, it did, but just barely...IMO..
No returns on used items, All sales final, and I was 6 states over, and they knew I was over a barrel.
Fast forward and it's 2024, I need a 2x12 combo amp, and held on to it all these years, thinking I could re-purpose this someday at a lesser capacity....
Reading some reviews from Reverb and other used gear sites, most say that the 8008 amp in general, should indeed produce more power than mine seemed to ever have.
Today, I just need this 8008 to drive 2 12" 16 Ohm 100W "KICS" speakers(another story) that I have laying around ( wired in parallel).
Expecting to get 8 Ohms on Channel B, only using Channel B for in / out connected to these two 12's...
Channel A to be unused.
I powered this amp up for first time in 10 years. It's been properly stored in a cool dry basement.
Red power light came on, great. It worked for two minutes with some humming (still sounded weak, but only producing 40W at 8 Ohms right?) ...then.....Pop and smoke!
Blew both channel fuses, but not the main. Channel A had no input or output.
I was running a Hotone Ampero into the channel B 1/4" input with substantial gain coming into channel B.
Now, I am waiting for new T6.3A 250V fuses to arrive, but what else should I check for in the meantime?
I am looking around for replacements for the two large blue bulged capacitors.
They seem to be SAMWHA CE-HC 85°C 50WV, 30mm x 30mm capacitors.
If I had to guess where they are, it would be at C11 and C14 in the PDF schematic, listed as 4700uF, and 50V.
Picture here for reference. The rest of the internals look OK to the naked eye (no burnt traces or components).
I have a DVM, but am unsure of what the issue is.
Many thanks in advance,
George

Hi. I have one of those 8008 since the 90s, and can tell you that it definetely has enough power to drive one or two Marshall cabs with 75s very LOUD, no issues there. It sounds pretty well, a little mid heavy compared to an all tube power amp, but VERY clear and detailed. The smoke could have been from diodes from the power supply or output transistors, definItely check those. Also the electrolytic caps are prone to fail, mine had to be changed, tricky to find the ones that fit in one rack space... All in all, is a great power amp if you can live without "presence" control.
One last thing: the "valvestate" thing works as it's supposed to work only into 4 Ohms loads. Trust me on that. In 16 Ohms it'll sound "meh" to be fair.

 

[Georgio_P]

I also testing all 6 output transistors in diode mode, placing the red lead on the middle leg and the black lead on the #3 (right most) leg.
I read continuity on one of the BVD64C Darlington transistors and on the BDV65C with the Marshall logo on it.
These were tested while on the board,
Please let me know if that means that these two output transistors are shorted or not.
Thank you,
George

 

[g1.1]

I read continuity on one of the BVD64C Darlington transistors and on the BDV65C with the Marshall logo on it.
These were tested while on the board,
Please let me know if that means that these two output transistors are shorted or not.

Continuity may mean different things depending on the meter. Numbers are best. Maybe it's a beeper? Some beep for less than 40 ohms, some under 200, etc.
But if 2 gave continuity and the other pair did not, then they are probably bad. You can remove them and re-check.

 

[Georgio_P]

Thanks, what do you think about the Rectifier Bridge? Red was on (+) and black was on (-) and I get continuity, and a tone.
I just see some fluctuation on readings between (-) and (S), and between the other (S) and (+).
Does this continuity between (+) and (-) legs in diode mode mean that this rectifier bridge is shorted internally?

 

[g1.1]

Does this continuity between (+) and (-) legs in diode mode mean that this rectifier bridge is shorted internally?

It may be due to the power transistors. Once they are removed, re-check the bridge.

 

[Georgio_P]

Screw it.
I am just going to replace the rectifier bridge and all 6 output transistors. I was able to order all from Mouser + 3-day shipping under $20.
I figure I might as well replace them if I am going to de-solder them from the board anyways being a 30+ year old amp at this point.
The only thing is .. Do I need to replace the (2) BDV65C's with the TIP142's and the (2) BDV64C's with the TIP147's in those same locations? The (2) TIP29C's are in the middle, so I will keep in same location.
Then going to power up with nothing in and nothing out connected. If nothing blows, then off, then hookup 8 Ohms load into Channel A output, and guitar input into Channel A only.
Thank you,
George

 

[danfrank]

By the way you described getting continuity with the red lead on the (+) of the bridge rectifier, that rectifier is shot. Typically what happens is the filter caps go bad and that takes out the rectifier. Sunwa (or whatever the brand was) suck. Those caps don’t last long.
Output transistors usually need to be taken out of circuit to test properly as the low value resistors connected to them will give false meter readings.
If you do switch out the output transistors you will probably have to reset bias of the output stage (transistors) by adjusting the small trimmer on the board that sets bias. I’m not familiar with the 8008, so the trimmer may not be there, but most transistor amps will have an adjustment trimmer.

EDIT: It looks like there's no trimmer to adjust current on the 8008 so disregard what I said above about adjusting.

 

[g1.1]

Do I need to replace the (2) BDV65C's with the TIP142's and the (2) BDV64C's with the TIP147's in those same locations?

Correct.
When you power up with no speakers/load connected, make sure there is no DC voltage at the outputs.
If there is any more than 50mV or so DC (0.05VDC) do not proceed, there is still something wrong.

 

[Georgio_P]

All the parts arrived today and were soldered in place. Progress was definitely made!!
The 8008 no longer blows fuses upon power up!
The LED glows, and there was no strange DC output noticed at output A with the DVM.
Channel A had guitar input and 8 Ohm speaker load on output.
Started playing it. It is working, but now it sounds well, ...like crap.
The sound it produces is like when the speaker voice coils are blown.
It is a very shallow, very tinny, distorted, and muffled, all at the same time.
No cracking/crackling, no hum, and output only when played as expected.
The output level seems OK for an 8 Ohm load.
I am guessing that one of the replacement components I used is causing a lot of distortion in the output signal chain.
I tested speakers again with a different amp, and they are great.
I don't think that its the bridge rectifier.
My guess is that some capacitors and output transistors that I used as replacements, even though the specs are the same as what was there, are causing signal chain distortion or anomolies.
A couple of things to note:
1. The caps C9 and C10 when the specs are the same, but the new ones are about 2mm shorter than the ones that were there. and I don't believe that they are considered "Audio Grade".
2. The BDV64C's were replaced with TIP147's, but , the TIP147's are about half the size of the BDV64C transistors, so much smaller that I had to spread the legs out to fit onto the board a lot. and they barely made it
3. Tried the 8008 both in Linear and Valvestate modes, and that did not matter in this case.
Can you please help point me in the direction of which component might be suspect in causing it to sound like crap this case?
Best Regards,
George

 

[g1.1]

2. The BDV64C's were replaced with TIP147's, but , the TIP147's are about half the size of the BDV64C transistors, so much smaller that I had to spread the legs out to fit onto the board a lot. and they barely made it

I think you must mean the 142's ?
I see on your order sheet TIP142T. Sorry I did not notice that earlier, the T suffix is for a smaller, lower power part.
You will want the 142 with no suffix, mouser # 511-TIP142.

As it is working, I don't think that is causing the sound problem all though it will fail early so they must be replaced with the larger parts.
Meantime you can continue with speakers disconnected.
First check that TR6 and TR15 are ok, you should be able to use the diode test without removing them. Just looking for any shorts.
They will sometimes fail as they protect the output transistors from over-current. Check D1 and D7 as well.

Then verify that the amp power supplies are giving the correct voltages, R11 should have around +40VDC at one end and +15VDC at the other. R12 should give same numbers but negative polarity.

 

[Georgio_P]

Hello G1.1,
I ordered the TIP142's.
In diode mode, D7 displays 597mV and D1 at 583mV touching one end with red and other side with black while on the board.
TR6 at 364mV and TR15 at 365mV when touching the middle and right leg in diode mode while on the board.
Let me know if these results make any sense.
I'll update in a couple days...
Once the newer TIP142's are installed, I'll proceed to test voltages at R11 and R12.
Thanks,
George

 

[g1.1]

In diode mode, D7 displays 597mV and D1 at 583mV touching one end with red and other side with black while on the board.
TR6 at 364mV and TR15 at 365mV when touching the middle and right leg in diode mode while on the board.

The diodes sound like they should be ok.
For TR6 and TR15, also check from pins 1 to 3 (outer legs).

 

[Georgio_P]

Yes, the outer pins forTR6 and TR15, I did check from pins 1 to 3 (outer legs).
What I observed was 709mV reading on TR15 legs 1 and 3 and 652mV reading on TR6 legs 1 and 3 in diode mode.
Also noted that these are mostly obsolete components, but Mouser lists similar ones with an "A" or a "B" at the end of the model. I am hoping that these are OK. though. Please let me know what you think.
Thanks,
George

 

[Georgio_P]

Hello,
I soldered in the new (Full sized) TIP142's, I am re-pasting the heat sink every time a Darlington is replaced with new thermal paste.
I also bought an ESR meter and tested all 8 capacitors on the board, and they are all in spec for ESR.
Here is the unit I purchased and am using https://www.amazon.com/dp/B00GYSFOM6?psc=1
When powered up, I tested the DC voltage on R11, and it was +40V one side, and +15V other side
R12 has same readings, just reversed like you said.
The output from channel A still sounds distorted and weak.
I read a similar thread Here: https://www.marshallforum.com/threads/problem-with-marshall-8008-power-amp.103714/
Says to check Z1 and Z2 (Zener diodes) for 15V DC, - and those are reading fine when power is on the board.
Same thread said to check IC1 pins 1 and 7 , should have no DC current.
On my IC1, I am reading +0.55V on Pin 1 and +13V on Pin 7.... (Pin 1 being the one with the dimple over the pin).
Would seeing Voltage on these pins explain the distortion in Channel A?
Markings are TL072CP KKBB 9313 , with a Motorola Logo on it....
Mouser # 595-TL072CP/ Digi-Key 296-1775-5-ND, seems to be the closest match...even with Texas Instruments markings on them.
Please let me know what you think.....
Thanks,
George