Marshall SV20H Pre-amp Issue

ryanverbena

I've recently experimented with my studio vintage by changing out the EL34's for some KT66's I had on hand, and swapped the stock 12ax7 in V1 for a tung sol 12ax7 just to see what happened. The change in tone was magnificent to say the least, and being cathode biased I was under the impression this would come without consequence.

2 days later, the amp spat and crackled when I turned it on, then proceeded to work but with very little volume. I thought it must have been the KT66's so quickly swapped them back with no improvement. Then I troubleshot the pre-amp tubes one by one to discover that it was one of the two tubes I'd left in. Not sure which one but replacing both completely sorted my problem. I've attached a photo of the two in question, one of which looks like it is definitely the culprit.

My questions to you experts are:

1) Could changing to KT66 tubes in such an amp have caused this problem in the pre-amp section, or is it likely coincidental that this happened when I changed them?

2) Is there any way to determine the cause of the damage by the visual deformation of the tube in question?

3) What would you advise I do now? I really, really preferred the tone with the KT66 valves but don't want this to be a regular thing.

Many thanks in advance!

[img]https://i.imgur.com/5vgdycc.jpg?1[/img]

[img]https://i.imgur.com/2fTqvRW.jpg?1[/img]

ICBM

ryanverbena said:

1) Could changing to KT66 tubes in such an amp have caused this problem in the pre-amp section, or is it likely coincidental that this happened when I changed them?

No. Complete coincidence.

ryanverbena said:

2) Is there any way to determine the cause of the damage by the visual deformation of the tube in question?

No, not usually. Those two look normal given the awful quality of modern valves! You can normally only tell if they've failed when the tops go white.

ryanverbena said:

3) What would you advise I do now? I really, really preferred the tone with the KT66 valves but don't want this to be a regular thing.

Replace whichever of the preamp valves is faulty - the chances are it's the middle one since that is a cathode-follower position, which can cause failures with Russian-made and sometimes JJ valves.

The valve on the left in the pic is Chinese and the one on the right is a JJ, so if it's not the Chinese one that has died, put that in the middle position. Tung-Sols are Russian so don't use one of those - they're fine in the first and third positions.

ryanverbena

That is very reassuring, many thanks for your detailed breakdown.

I have a bunch of new JJ's so will move the Chinese 12ax7 to V2, keep the tung sol in V1 and a JJ in V3. I didn't know their interactions would be so nuanced! Cheers again for the wisdom. Will let you know how it goes  

digitalkettle

So you’re sticking with the KT66s then?

ryanverbena

I'm going to do a bit more research, but it would be such a pity to not keep with it because of a (possibly non-related) pre-amp tube defect.

All accounts I've read on other forums seem to show unanimously positive outcomes from changing the power tubes..

digitalkettle

I got an SV20H just a few weeks ago. It's been out a few times with positive results.

I'm interested in your findings!

Wizik

@ICBM Can this el34 to kt66 valve change be applicable to the Origin 20H as well? Is it right that they are similar by design? thanks

ryanverbena

digitalkettle said:

I got an SV20H just a few weeks ago. It's been out a few times with positive results.

I'm interested in your findings!

I'll be sure to keep you posted, fantastic amps! What cab are you running? I'm playing mine through a 2061CX with G12H30's and love it. 

I also have a 89' 1962 bluesbreaker RI which has a pair of really well broken in greenbacks. Was thinking of trying them in the 2061cx to see how it sounded, whilst trying some celestion alnico golds in the 1962.

Fun times ahead!

ICBM

Wizik said:

@ICBM Can this el34 to kt66 valve change be applicable to the Origin 20H as well? Is it right that they are similar by design? thanks

I'm not 100% sure - if the Origin is also cathode-biased then it will be. I think it is, but having not seen inside one yet I can't positively confirm it.

Ossyrocks

ryanverbena said:

digitalkettle said:

I got an SV20H just a few weeks ago. It's been out a few times with positive results.

I'm interested in your findings!

I'll be sure to keep you posted, fantastic amps! What cab are you running? I'm playing mine through a 2061CX with G12H30's and love it. 

I also have a 89' 1962 bluesbreaker RI which has a pair of really well broken in greenbacks. Was thinking of trying them in the 2061cx to see how it sounded, whilst trying some celestion alnico golds in the 1962.

Fun times ahead!

Alnico Golds in the Bluesbreaker will be LOUD. They are more efficient than the greenbacks I think. I put a gold in a 1974X, it was louder and tighter.

Rob

digitalkettle

ryanverbena said:

I'll be sure to keep you posted, fantastic amps! What cab are you running?

I haven't got a dedicated cab yet so I was running it through a Dr Z 1x12 (open-backed) with a Celestion Gold.

I quite fancy getting a 2x12 with Creambacks.

ryanverbena

Alnico Golds in the Bluesbreaker will be LOUD. They are more efficient than the greenbacks I think. I put a gold in a 1974X, it was louder and tighter.

Rob

That's not ideal actually! It's already plenty loud enough.

Do you think the volume difference will drown out or dilute the sound of the greenback too much? In that case it may be a bad idea... 

ICBM

What I would do is swap one of the Greenbacks for one of the H30s. Put the Greenback at the top in the 2061 and the H30 on the left (opposite end from the power transformer!) in the Bluesbreaker. Best of both worlds, they balance perfectly for volume and it costs nothing.

ryanverbena

I already own the celestion gold, it's currently not being used at all so thoight I'd give it a shot 

ICBM

If you’ve just got one, try it on it’s own in the Bluesbreaker - it will handle the power fine, and help keep the volume down. The empty hole from the other one doesn’t matter.

ryanverbena

Not sure that's for me. I used to own a 1974x which was incredible, but next to the Bluesbreaker it just didn't cut it for me - Jjst felt boxy (I know plenty of folks would say the opposite). I also realise cab size will have played a role in this. If anything I'd probably rather wait for another second hand alnico gold and run them together in the Bluesbreaker. 

The idea of running a greenback and a g12h30, or maybe both greenbacks, in the 2061cx with the SV20h is something I'll definitely be trying out!