• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tips requested for tweaking this valve amp.

el donkey

Member
2014-10-06 9:12 am
I have a Beard P50 power amp. Its part of an all valve setup feeding a pair of rebuilt Gale 401s

The P50 was aged when I got it. I first recapped most of the signal and all of the psu sections.
Bypassed the input level pot
Fitted Mazda tubes and GEC KT88s

It has ran for a few years of light use since (tubes don't have many so hours on them).

It does still have some low level hum but not sure if this unavoidable. It can be heard when close to the speakers with no signal playing.

I previously looked at this, added a balance pot as per instructions on here to the AC heater lines. Adjusting that did not do much.
I also fitted a few diodes to lower the heater voltage by 0.6v as due to running an isolation transformer my incoming mains is a shade high.

Not sure what the source of the hum is.

I will be taking another look at the amp in the coming weeks. I still need to fix the panel lights but have not figured an easy way to access those as sandwiched between front panel and chassis. Not sure if removing the rack mount handles and all panel knobs will release the front panel or a chassis strip down is required. Lots of hidden screws that would be invasive to reach.

I have new diodes ready to replace the 240v input rectification. Just as an age replacement and better spec.

Any aged looking resistors I replaced when did the caps.

My next thoughts are the signal wires.
These run from the rca's at the rear to the front of the chassis. They are solid core, layed neatly on sticky pads but not twisted.
I will check to see if the path can be improved/shortened.
Thoughts on these please,
try new twisted signal wires? If so what type.
I want to order any bits I may require first. I will then post pics for any further advice. I may have some earlier ones I can dig out.

Another mod I was thinking was to add some extra caps to the large psu items I replaced. I gather the P80 version of this amp had these but have no diagrams for either despite several attempts to find any. The Gale 401s like some grunt to wake them up so maybe lots of reserve in the large caps would be useful.

All the tube sockets are original. I nipped in the female legs rather than replace last time. I don't get any of the rustling leaves sound so assume they are not a weak point sonically as is.

Thanks Rob
 

el donkey

Member
2014-10-06 9:12 am
Ok, I will try and search for some photos of the internal layout and attach here.
That may give experienced amp builders an idea whether the existing rca to pcb wiring is as good as it can be.

I will probably bypass the A/B/mute switch as only use one of the amps two sets of rca's. I will also fit some new rca sockets with basic copper/gold items.

I have done a search on the best signal wire to run from the rca inputs to the first stage pcb.
Unfortunately the threads I see are somewhat messy with the usual 'red sounds better' comments.

So can an experienced builder answer?

A. The existing untwisted wire is fine as is.
B. Twisted might be better
C. Screened is the way to go (with the screen earthed to chassis at the input end only?)

Thanks.
 

el donkey

Member
2014-10-06 9:12 am
some photos
 

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el donkey

Member
2014-10-06 9:12 am
The amp uses ECC83 - ECC82 - 2 x KT88s per channel.

looking at the photo's the wiring looks a bit scruffy compared to some of the self builds I see.

The input signal appears to go from the RCAs - to the power off switch at the front panel - to the A/B/Mute switch - to the pcb. The input sensitivity pot has already been bypassed.

Is there a reason the input signal needs to be switched along with the power on?

Would I be better using some basic Van Damme 2 core screened cable direct from the RCA to the PCB raised on some pegs directly down the centre of the two banks? As stated I only use one set of inputs so no need of the A/B switch. The second set of RCAs have lowering resistors which were there for a CD player input when the input sensitivity pot was being used instead of a pre amp. all sources now go via a vale pre amp into the other set of RCA inputs.

I wish to do the following.

1. Fit new RCA sockets as the existing are a bit iffy at times.
2. Reduce any possible causes of hum
3. increase the PSU four large electrolytic capacitors by adding some in parallel (based on the theory the P80 used larger values)
4. Fix the dead panel lamps
5. Any other suggestions while I have it open? The amp does sound very good as is, my only remark on sound quality is it does sound better when turned up. Bass or weight can be a bit lacking a lower volume

Thanks, hope I am not talking to myself :)
 

el donkey

Member
2014-10-06 9:12 am
Nice amplifier , first you should find the hum frequency , if it's mains frequency 50/60Hz from unshielded cables , filaments so on , or double frequency from B+ not filtered good enough .
I can put an oscilloscope onto the speaker terminals and take a photo. Is it best to put the probe on the +ve and the earth clip to the -ve post or chassis earth to get the most accurate view?
Thanks Rob
 

Zung

Member
2005-04-30 7:30 pm
Geneva
Found some sort of schematics here, amp section shown, the rest on the web page. Should be close to your amp, but beware of the mistakes.

1642940033791.png


Regarding the hum, there're a few options:
  1. Tired PS caps: you said you want to do a cap job anyway, so go ahead and do it, plus:
  2. The bias cap (47uF 100V) looks wimpy: I'd replace it with a big one, but no more than 470uF for safety. It also supplies the tail of the input.
  3. Run the input LTP heater with DC: move the ground to the lower leg of the heater winding, then a CRC filter with 1000uF and 1R. I did this to my Fender Champ guitar amp to eliminate the hum, totally
  4. Shielded cable at the input - Get a good one
In general:
  • Coupling caps: use MKP at the strict minimum, or exotic snake oil types at your discretion. Ignore those who say they sound the same
  • Add decoupling caps to all PS caps, same type as above
  • Rectifiers: I use Schottky's only
 
Found some sort of schematics here, amp section shown, the rest on the web page. Should be close to your amp, but beware of the mistakes.

View attachment 1017897

Regarding the hum, there're a few options:
  1. Tired PS caps: you said you want to do a cap job anyway, so go ahead and do it, plus:
  2. The bias cap (47uF 100V) looks wimpy: I'd replace it with a big one, but no more than 470uF for safety. It also supplies the tail of the input.
  3. Run the input LTP heater with DC: move the ground to the lower leg of the heater winding, then a CRC filter with 1000uF and 1R. I did this to my Fender Champ guitar amp to eliminate the hum, totally
  4. Shielded cable at the input - Get a good one
In general:
  • Coupling caps: use MKP at the strict minimum, or exotic snake oil types at your discretion. Ignore those who say they sound the same
  • Add decoupling caps to all PS caps, same type as above
  • Rectifiers: I use Schottky's only
Thanks, The circuit for the P100 is differs to the P50 but the design theory is probably similar. Shame there is not the same information for the P50 but I have tried a number of times to find anything more.

The P50 had 350v 600uf reservoir caps as new but those were leaking so I changed them for the Kemet 385v 560uf previously. The P80 which I gather was the same circuit but had 800uf caps or 2 x 400uf. I gather it had even more grunt.

I was thinking of adding 2 x 100uf 450v Rubycons to each of the 560uf I have and also a film 0.1uf. 2x 100uf will fit easier and depending upon what I read, having a combination of sizes can help with the resistance and recharge speeds? That will give me 760uf x 2 on each bank so close to the 800uf of the P80.

I will have to look when I open the P50 up again to see if the 47uf you refer to is the same. Could that be the gold electrolytic next to the white ceramic resistors in my photo?

Unless there is a differing reasoning not to proceed with the planned additional caps I will get my parts ordered and complete the following then assess.

1. Extra capacitors as above
2. Replace the now iffy RCA input sockets
3. Shorten the input signal path direct to PCB with screened 2 core
4. Attempt to access the failed panel lamps
 

Zung

Member
2005-04-30 7:30 pm
Geneva
...
I will have to look when I open the P50 up again to see if the 47uf you refer to is the same. Could that be the gold electrolytic next to the white ceramic resistors in my photo?
...
Not sure - Another candidate could the the black axial thingies L & R side, marked 33uF. You'll have to trace. Also, I'm not sure about the additional Rubycon cap you've planned; I'd rather use big MCaps (1uF?), they're not that $$$.
 
Not sure - Another candidate could the the black axial thingies L & R side, marked 33uF. You'll have to trace. Also, I'm not sure about the additional Rubycon cap you've planned; I'd rather use big MCaps (1uF?), they're not that $$$.
What are your concerns on the additional Rubycons? my thinking is that if the Beard P80 version of this amp could make use of the extra reserve then the hungry Gale 401 speakers I have may appreciate it also.
In terms of adding a 1uf if you think that is beneficial to smoothing the supply I have some good K73-16B 400v caps I could also add.
 

Zung

Member
2005-04-30 7:30 pm
Geneva
Extra "reserve" is a good thing, but adding 100uF to 560uF (+20%) is probably not worth the trouble.
Adding a 1uF film cap will not change the smoothing per se, but will change the "voicing" of your amp. I've never heard any PET caps like yours, but they're fairly close to polyester caps (evil), so I'd avoid them.
 

el donkey

Member
2014-10-06 9:12 am
Extra "reserve" is a good thing, but adding 100uF to 560uF (+20%) is probably not worth the trouble.
Adding a 1uF film cap will not change the smoothing per se, but will change the "voicing" of your amp. I've never heard any PET caps like yours, but they're fairly close to polyester caps (evil), so I'd avoid them.
Those K73s are amazing as output caps for say a preamp or power amp that does not use a transformer out. Better than mundorf silver or golds.
I once spent months burning in on a rack about 20 different types of both the russian and expensive western varieties.
I then rigged up quick swap posts and did back to back tests.
In terms of use on a PSU section I have no idea, my knowledge of psu design is limited.
The idea of adding the rubycons would be 2x100uf to each of the four 560ufs
Giving me a jump to 760uf on each.
I could i guess alternatively try a 220uf and a 100uf on each to go a bit further.
Its all guess work on my side really.

How would a 1uf onto the psu stage effect the voicing?
Rob
 

el donkey

Member
2014-10-06 9:12 am
What are you trying to achieve here? Isn't your first goal to eliminate the hum? Improving the circuit by adding boutique parts, or parts with different specs, will just complicate the picture.

I am assuming that the amplifier would not have had the hum you mentioned when it was new.
The hum is not 'bad' and may well be as was when new. It might be just typical of such a valve amp but it won't hurt trying to reduce it.
The purpose is as per my list above so I am just looking into what else I may as well do while I have it open again. It is a seriously heavy lump so doing work in one hit makes sense.

Cleaning the input cable path just makes sense I think and will cost nothing but the short piece of screened cable.

The RCA sockets are a must step as the existing sometimes need a wiggle which is not good.

Adding some extra capacitors I believe will just follow the design of the Beard P80.

None of the intended parts are silly priced. The rubycons do not cost a great deal and I have lots of other capacitors in my spares box to use if might do any good.

The fact the lamps are dead bug me a little but not enough to have bothered with until now, due to the sockets though its time to do a service.