What to hell i'm doing bad (SC 200+)

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What to hell i'm doing bad (SC 200+)

Postby Harvey58 on Wed Mar 26, 2014 11:42 am

Its about 8 years im using SC200+ converted to Hiwatt preamp and i dont have any troubles.
Sometime i do some improvement on preamp to changing the sound a little. Im using it for bass.
When i got this beast, it was set and biased for 4x EL34's and it plays OK,
but i decided to revert it to the original output with 4xKT88. It was done and it works for 8 years how its said.

But you know it, sometimes ones want change something, so i want to try put a el34 back again.
I change BIAS in this way:

1/ On SC200+ there is 100 ohm resistor right behind the PT bias winding. I put 1k there.
2/ After this mod i can set the negative voltage to -36V (range for e34)
3/ I have new matched quartet of JJ EL34
4/ The amp have big plate voltage (ca 740V) but the EL34 can handle 800V, so i thing this must be in ball park
5/ I did measure plate current with probe (1 ohm resistor between cathode/groud) and set the BIAS to ca 23mA
(70% of 25W max EL34 plate Wattage) so i hope i did it right

Then i run the amp and all it seems to work, no red plates no extra glowing. I play bass few hours and all was OK.
After 2 days i run the amp, play few licks and one valve goes red plating. Off amp, check bias voltage (-36 on all valves)
run again and the red plating valve make big white light. Again stoping the amp, starting, filaments are OK, the lamps glowing normaly.
So i pull up the stand by, nothing happens and there is no sound.
When i check amp voltages without valves, everythink is OK. All bias voltage is -36, all plate voltage is 740. Screen voltages is about 420V or so.

Any idea what im doing wrong? The Valve, improved bias winding, or i set the valves to hot???
Next days i will change the bias resistor a put back the KT88, but the el34 were playing sooo nice!

Thanks.
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Re: What to hell i'm doing bad (SC 200+)

Postby Unit_1 on Wed Mar 26, 2014 4:15 pm

wow, that's a pretty high voltage! but the 23ma comes out right...

do you have any way of testing the tubes?

just recently i biased a pair of EL34's in a hiwatt dr504 clone and started playing. after 10 minutes I hear what sounds like a digital running water effect on the sound followed by a bright flash of a fuse. peeked inside in time to see one tube red plating just before it cooled off.

i double checked the bias, one was still running where I had set it, at about 33ma (504v at the plate), the other was running at about 54ma. both had the exact same negative voltage though (checked that first since I had just added a trim pot to the circuit to make it adjustable)

brand new matched, selected TAD set!!!

i put the red-plating tube into my tube tester and it was BAD - S.O.L. about $17 down the drain.

took another pair out of my tube drawer and everything was fine.
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Re: What to hell i'm doing bad (SC 200+)

Postby Harvey58 on Wed Mar 26, 2014 9:26 pm

No, i cant check it, i dont have any tube checker. They were bought as MQ and matching was did by manufacturer (JJs are near by, im from Czech and JJs are Slovak).
But i think one, maybe two of them are bad now :( Today im looking at the tubes and one pair is very rattle inside, so maybe some mechanical reason?

But still im asking to anyone who have experience - its possible to play EL34 at this high voltages in SC or simply not?
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Re: What to hell i'm doing bad (SC 200+)

Postby Unit_1 on Wed Mar 26, 2014 10:05 pm

http://www.drtube.com/datasheets/el34-sed1997.pdf

this spec sheet says they can take up to 800v :shock:
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Re: What to hell i'm doing bad (SC 200+)

Postby Papa Dog on Thu Mar 27, 2014 12:09 am

Harvey58 wrote:But i think one, maybe two of them are bad now :( Today im looking at the tubes and one pair is very rattle inside, so maybe some mechanical reason?

"very rattle inside" is a fair good indication that the tube in question is damaged and not useable.
tubes do fail and sometimes come from the maker DOA (dead on arrival). Whether that one was killed by issues with the amp or was just a bad tube is hard to say
But still im asking to anyone who have experience - its possible to play EL34 at this high voltages in SC or simply not?

Seems that you could run EL34s, but why???
Those amps were designed to breath correctly with KT88s and get along well with 6550s, but you are giving away half your power capacity by running EL34s...might be desirable for someone playing guitar and wanting some power tube distortion, but not my idea of a good thing for bass at all.

Pitty that amp was pillaged for its bits, as I find the Mk4 pre-amp to be more versitile and usefull than ANY passive pre-amp out there.
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Re: What to hell i'm doing bad (SC 200+)

Postby Mr. Foxen on Thu Mar 27, 2014 2:51 am

Harvey58 wrote:Any idea what im doing wrong?


Harvey58 wrote:SC200+converted to Hiwatt preamp
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Re: What to hell i'm doing bad (SC 200+)

Postby Harvey58 on Thu Mar 27, 2014 1:22 pm

:mrgreen:
Its Funny how people (so i do) have presumptions, but much of them dont have experiences in particular case.
I got this amp with moded preamp and have not clue how it is done. It was active preamp, but not in the original order. And the active preamp realy sucks! So i decide to mod it to Hiwatt preamp, because of similarity with Hiwatt.
Year by year i little tweek some components, so the preamp is not exactly hiwatt, but something between hiwatt and marshall and i like it. But sound of the power amp is too much BIG, not only loud, but too much wide (headroom) and the KT88 is also very glassy valves, which makes a little cold sound. The EL34 in it as i wrote (till the valves works) has beautifull sound, warmer and less BIG, but not as quiet as you decide. The loudness is not done only by wattage. Loudness were perfect, and the amp was not so boomy. I think the EL34 suits the bass amp more than KT88 and mainly in this amps (hiwatt, sound city).
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Re: What to hell i'm doing bad (SC 200+)

Postby Papa Dog on Thu Mar 27, 2014 5:27 pm

Harvey58 wrote::mrgreen:
Its Funny how people (so i do) have presumptions, but much of them dont have experiences in particular case.

at least two of us here have vast experience with the Mk4 pre-amp. I think that I know about as mucha bout how they work as anyone here, and Mr Foxen has likely re-built more Mk4 amps than anyone I have heard of.
I got this amp with moded preamp and have not clue how it is done. It was active preamp, but not in the original order. And the active preamp realy sucks!

There ya have it in a nutshell. sometimes folks will mod something without know how it works or why it is built the way that it is. So, randomly changing the values of any component in the "ladder" circuit might give one un-predicatable or random results, which are most likely not going to be what the user wants. You might rightly say that you do not like "the" or "a" active tone control, but to say that the Mk 4 active tone controls "sucks" is just not right. I happen to like the Mk4 active pre-ampo...but then, I understand it. Might be that a better shoice for you would have been to put it back stock before you decided to remove it entirely...you might have actually liked it.
So i decide to mod it to Hiwatt preamp, because of similarity with Hiwatt.
Year by year i little tweek some components, so the preamp is not exactly hiwatt, but something between hiwatt and marshall and i like it.
Which is what usually happens...folks know one or another amp, like it, and decide to make a Sound City sound like something else. Yes, you can do away with the active tone circuit and replace it with something more "conventional" and get something intersting, but it is then neither a Sound City nor a HiWatt nor a Marshall.
But sound of the power amp is too much BIG, not only loud, but too much wide (headroom) and the KT88 is also very glassy valves, which makes a little cold sound. The EL34 in it as i wrote (till the valves works) has beautifull sound, warmer and less BIG, but not as quiet as you decide. The loudness is not done only by wattage. Loudness were perfect, and the amp was not so boomy. I think the EL34 suits the bass amp more than KT88 and mainly in this amps (hiwatt, sound city).

The idea was/is for that amp to be "big". It IS big. Making it into something it was not designred to be might not be the best use of resources.
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Re: What to hell i'm doing bad (SC 200+)

Postby Harvey58 on Thu Mar 27, 2014 7:29 pm

Eh, sorry, dont misunderstand me. I dont mean it bad, and if it sounds offensive so im apologies for that.

I cant tell that i dont like any active preamp, but this active preamp was not much useful for me, so i change it. Hiwatt was logical way how to do it.
For example active preamp used in Ampeg is nice, not perfect, but very interesting and is calculated perfectly. Not tweeked randomly as many early passive was.

The Hiwatt preamp with some RC tune works fine (and is measured and calculated with SW utility like Multisim or Duncan ToneStack) but the preamp is not what i need to solve.

About the experiences, did you try to change the valves in that way? Did you hear that sound? If not, that's what I said :)
Many amps were moded to little another usage than how they were designed. David Reeves also tweeks the SC amps in early days for the guys from the WHO etc.
So im not as conservative in this way. But im simulate the changes prior i do change. Unfortunately power amps cannot be easily simulated.

Thanks for opinion.
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Re: What to hell i'm doing bad (SC 200+)

Postby Harvey58 on Wed May 28, 2014 1:45 pm

Finaly problem solved.
The more I think about it, the more I think the main trouble was loose on valve socket.
EL34 have thinner pins than KT88, so the loose socket makes one valve of the pair not connected at a moment, so the second in pair was then overheated to death! I did tighten the socket contacts and i also use Belton Tube Retainer. The tubes sitting in socket perfect now.

Finaly i use JJ E34L, they works nice, with a sound i like.
For using EL34 tubes i place 1K/5W screen resistors (rather than previous 100R) and also change the BIAS leak resistor (4k7 over ground in BIAS winding) > now im using 10K trimmer, so i can change the BIAS range for both kind of valves.

Again, the EL34 sounds really nice for bass :jam:
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