advertisement


Amp Surgery. Worthwhile or not?

Mullardman

Mullardman

Moderately extreme...
Apart from Ohms Law and a fair idea of the difference between a capacitor and a resistor, my electronics knowledge amounts to the square root of f**k all. ( A charming analogy picked up from a friend, who is a maths grad)

Can anyone tell me...

1. How difficult/otherwise, would it be, to take a 100w push/pull EL34 valve amp and 'triode strap' the output valves.?
2. Roughly how much power would result?
3. What effect would this have on the sound?

which leads to..

4. Would it be worth it?

Mull
 
if you don't know .....ask yourself if you should proceed without further research/learning

the voltages in valve gear can kill ......

you don't say what the make of the amp is ....
 
zanash said:
if you don't know .....ask yourself if you should proceed without further research/learning

the voltages in valve gear can kill ......

you don't say what the make of the amp is ....
Click to expand...

Zanash,

My post was the first part of my research. I don't know much about circuitry, but neither am I stupid. I know the voltages involved. I know the procedure for a), minimising residual voltages in caps, and b) checking what's still there before doing work. I also know how to avoid shocks, and especially how to avoid them 'across the heart'. My heart is already damaged.. I don't take stupid risks. I have successfully fitted by-pass caps to my amps, set the valve bias, and fitted an earth-lifting mod on a similar amp to eliminate buzz. Most of this with advice and clear instructions from the manufacturer, but done by me.

However, I didn't say this work was going to be done at all, never mind by me. I was simply asking for ideas/information.

I have Papworth 100 watt monoblocs., EL34's in push pull. I have more power than I need, and am interested to find out precisely what I asked.

There seems to be a concensus that triodes sound 'sweeter'. I presume that triode strapping EL34's might do this. I also presume that this will result in a lowering of power output. I was merely enquiring by how much.

There may be other considerations which make this idea impractical, or pointless. I don't know.. which is why I asked.

Regards,
Mull
 
Sorry didn't mean it in that way .......

in fact what you are proposing is what I meant to say

But you don't mess with valve circuits lightly

perhaps you implied less knowledge than you had in the original post .....

I've no experience with the papworth amps nor am I familiar with the circuit, but I have modded valve gear.

If you have not looked at the rectification ...that could provide you with some of the sweetness your missing [it did for me] my unit had solid state rectification so with the substituiton of ultrafast diodes a lot of the hash was removed from the psu supply rails.

Also look at the quality of the decoupling caps in the signal path ....I'm a great believer in paper in oil or similar....its staggering what the average blocking caps is restricting .....

but as to the triode mod I've not been that route though by all accounts its not hard to do.

remember to make certain the resovoir caps are drained before you start work ...
 
Hi Mull,

Output would reduce to about 40-50w going by what folks on the WD board have found.

IIRC it will also lower output impedance (usualy a good thing) an in theory would lower distortion.

I know that in the WAD and WD amp circuits it is also necessary t change the feedback arrangement for triode use. Perhaps pop over to WD where you'll get a full answer from some very wise folk.

http://www.world-designs.co.uk/forum/
 
Well chaps,

I took the blindingly obvious step of contacting the manufacturer of my amps. The salient points of his reply are quoted below.

'Ages ago, maybe five years, we did a job for ('a customer') with M100s, living in London. We took the m100s, and removed the "cross coupling" resistors, reduced the HT considerably by altering the way the main bridge rectifier works, coupled the EL34s as triodes (by taking the screen resistor directly to the anode), increased the cathode resistors, altered the cathode current to run in class A.

From memory, this gave 28 watts out in pure class A.'

Since this would cost me very little, and would presumably be reversible, thus preserving the resale value of what are a 'classic' amps, I'm highly tempted.

Also, at 28 watts, they'd work pretty well with my existing speakers, allowing me to evaluate them for not much more than beer money, before going off and buying anything exotic.

I'm still struggling a bit with a few ideas related to getting my head around the differences between 'push-pull', 'single -ended', class A etc., but I'll get there eventually.

I should explain my reasons for the above ideas.
Even with relatively insensitive speakers, I've never used more than a fraction of the output available from the Papworth mono's. I was labouring under the impression that plentiful power is never wasted, in terms of transient response etc. That was until I heard Andy's 2 watt SET amp driving his Tannoy GRF's. ( See 'My Tannoy's' thread, in Classic)
Absolutely gorgeous. A little underpowered, and not providing the sort of bass power and slam I like, but a fab mid, with great imaging and real atmosphere. My M100's drove his GRF's better in terms of bass power, but couldn't replicate the delicacy and finesse of his little SET. So, I started thinking about affordable ways of getting something similar. I clearly need to change my speakers, and in fact have been considering this for some time. But the amps seem to me to be the main issue.

Regards,

Mull
 
For part of your question, triode-strapping the EL34's is quite easy, wire a resistor between anode and grid 2 (pins 3 and 4 on the valve). Then all you need to do is disconnect the wire from G3 to the transformer, ist ages since I did it but its very easy to do. Im sure theres diagrams on the internet somewhere.

Ive tried it with El34's and I wasnt that impressed, seemed to lose some 'slam'. Maybe it depends on the map design.
 
Hi Mull

Many thanks for the kind words lol .... pay ya later .......;)

I have today substituted the RCA 5U4 (Rectifier tube) for a Used Mullard GZ32. Instantly there is more bass which has come forward in the soundstage and is more pronounced and coherant.

Then I changed the stock Svetlana`s SV 83`s (Output tubes) for a pair of Brimar EL84 Which has again changed the dynamics of the mix, things seem faster, and the image is better than it was before.

So I think I may actually have addressed some of the shortcomings associated with this little SET amp.

My next move will be to get another one, tubed in a similar manner, and strap them together to work as a Mono Pair which will, (if the maunfacturer is to be believed) increase the power output to 6 watts.

Regards

Andy
 
I've been reading loads of back issues of HFW supplement. There are a few who don't rate the idea of triode strapping pentodes. Old Tim de PVC seems to be one of 'em. OTOH, HFW did a 'EL 34' amp, using the Edicron/EI 6CA7, which is an EL34 equivalent, but according to HFW, has higher dissipation, deeper tauter bass etc. Might be worth a go in my 'new' Papworths. The other approach is, of course, to sell 'em 'as is' and use the cash to buy a proper triode amp. Decisions.. decisions.....
 
You must log in or register to reply here.


advertisement


Back
Top