Opportunity to buy an old valve guitar amp

[Bushbaby]

Just to double check, I am proposing to revert this amp to the black faced (bf) spec which is to drop the 40uf cap to the rectifier valve to 22uf (nearest to 20uf). The bf version operated at 50v lower at the rectifier which means the amp has slight less output, distorts earlier and sounds a little sweeter.
Any thoughts
Rob

[pwdrive]

Just to double check, I am proposing to revert this amp to the black faced (bf) spec which is to drop the 40uf cap to the rectifier valve to 22uf (nearest to 20uf). The bf version operated at 50v lower at the rectifier which means the amp has slight less output, distorts earlier and sounds a little sweeter.
Any thoughts
Rob

Rob, are the differences you quote above the only differences?
If dropping the reservior capacitor from 40uf to 20uf then bear in mind the wide tolerance swing of electrolytic values in general, you might have to select the capacitor using a capacitance bridge

Lawrence.

[Bushbaby]

Hi Lawrence
Looking at the schematic aa764 vibrochamp to ab764 bronco the only difference is the use of a 20/20/20uf cap to a 40/20/20uf cap. Very subtle difference indeed.

Rob

[Yorkie]

Given its age, before I used the amp, I’d replace the .02 cap between the anode of ½ 12AX7 (pin 6) that feeds the grid of the 6V6 output valve pin 5. (‘that cap’). That cap has 200V DC on the input – you don’t want any of that on the grid of the 6V6 – all you want is the AC signal. Then I’d replace the 25uF 25V cathode bypass on the 6V6 (from pin 8 to ground), because it will almost certainly have dried out. Modern ones (22uF 25V) are a puny size, so if you want to preserve originality of appearance, you might want to stuff the old cap, but you may actually find that a modern cap won’t look out of place as the existing one is mounted on the tag-board adjacent to the 6V6, so may in fact be quite small itself.

Personally, before I meddled with the smoothing/reservoir caps I’d reform them, check the leakage and ESR and if they’re good, I’d leave well alone for now.

When you’ve done that, plug in your guitar(s)! The most essential component of any sound system is our ears – see what it sounds like – don’t take for granted statements by self-styled aficionados, judge for yourself before tinkering with it. Statements such as ‘distorts earlier and sounds a little sweeter’ that aren’t backed up with any tests are entirely subjective should be treated that with scepticism. Has whoever made that statement explained at what frequency and amplitude does a sine wave distort on this amp when fitted with 22 uF caps, and how do the oscilloscope traces compare with that of 40uF caps? I think we know the answer to that – it’s just someone’s subjective view dressed up as ‘fact’ with no underpinning science to support it.

As to replacing the 40uF cap ‘to the rectifier’ to be pedantic, the reservoir/smoothing caps aren’t ‘to’ rectifier – they’re from it. The layout of the chassis at the link below shows a three section unit in one can at the end of the tag-board, but given the long run of this amp (19 years), maybe some had discrete caps. You might struggle to get a three section unit and the old can will be too small to stuff three individual caps inside Rob.

http://vibrochamp.org/wp-content/uploads...layout.jpg

When you say ‘the bf version operated at 50V lower at the rectifier which means the amp has slight less output’, that may well be so but I doubt it. The circuit of the AA764 using 20 uF caps shows 342V on the anode of the 6V6. 50V higher with 40uF caps? I don’t think so! (The circuit says the voltage readings should be taken with an electronic meter (ie NOT a 20,000 OPV AVO 8 or whatever, but a valve voltmeter, or more likely these days, a DVM) and are +/- 20% of stated values. Resistors are +/- 10%.

That's my ten penn'orth.

Hope it arrives by the weekend Rob!!

Have fun

[pwdrive]

So far as I am aware the difference in the HT between the two is due to a different mains transformer, one gives out 315 volts ac under nominal load and the other gives out 365 volts ac under nominal load.
Looks like direct cathode rectifier to me, if so then don't exceed the maximum reservior capacitance value, that value is usually stated in the manufactures valve data.

Lawrence.

[Robert Darwent]

I am proposing to revert this amp to the black faced (bf) spec which is to drop the 40uf cap to the rectifier valve to 22uf (nearest to 20uf). The bf version operated at 50v lower at the rectifier...

So far as I am aware the difference in the HT between the two is due to a different mains transformer, one gives out 315 volts ac under nominal load and the other gives out 365 volts ac under nominal load...

I'm not at all familiar with guitar amps, but looking at the circuit which seems a fairly standard output stage I have got to agree with Lawrence's comment.

Simply changing a reservoir capacitor by itself for one half its specified value is not going to drop the HT by 50 volts. The difference between the circuit diagrams of the two amps is far less "subtle" than it appears I'm afraid.

Regards

[boiss]

I saw 4 Guitar amps at the market today, the cheapest was 15 Euros, I don't remember the make, there was a Fender Princeton 112 with a first hit price of 110Euros, I asked whether it was valve or transistor, the guy said that he didn't know as he was selling it for a mate. the Amp was enclosed in a metal case at the top of the cabinet with part of the mains? tranny showing. I took a couple of photos, if anyone is interested I'll post them.
Mike

[pwdrive]

The one with the higher HT seems to be at or exceeding it's limits, 5Y3 manufactures recommended reservior cap = 40uf.
6V6 data would suggest the the high HT version amp is at or maybe exceeding the valves limits?

Lawrence.

[Bushbaby]

Hi All,
Having looked again at the circuits there are very some very subtle differences. It does appear that TR1 and TR2 are they same in both models. In the vibrochamp the valves are 12ax7's and a 6v6gt. In the bronco they are 12ax7a's and a 6v6gta. The circuits are identical.....apparently there is an error with voltage figures in the Bronco schematic (its too high). The only other difference is the 40/20/20 and 20/20/20 smoothing cap otherwise they are the same.

Regards

Rob

[pwdrive]

Could you post the two schematics up Rob? [EDIT: The two schematics I was refering too as regards HT differences were the Vibrochamp AA764 and the Bronco AB764, if they have made a mistake with the HT voltage on the latter then they have made a mistake on all the voltages in the schematic, I am reading AA764 OPV = 6V6gt, ac into rect = 315v, ht rect cathode = 355 v.
AB764 OPV = 6V6gta, ac into rect = 365v, ht rect cathode 420v.

Lawrence.