Mystery amplifier

[Kokotoni Will]

Originally Posted by PJL

V8 anode voltage is too low and that sets the bias conditions for V7 and that affects all the other voltages.

Swap the CV4006's over and see if the anode voltages change.

I've tested again with valves swapped, but there's no change.

[PJL]

C23 may be faulty. Remove V7 and V8 and measure the voltage on C23, it should be 250V.
If it is close to 250V then check R37 (39K) as it might have gone high.

[Kokotoni Will]

Originally Posted by PJL

C23 may be faulty. Remove V7 and V8 and measure the voltage on C23, it should be 250V.
If it is close to 250V then check R37 (39K) as it might have gone high.

It's 122V.

Would a couple of these make good replacements:

https://cricklewoodelectronics.com/33H450.html

Or would I be better off with something like this:

https://cricklewoodelectronics.com/32uF-32uF-500V-Dual-Electrolytic-Can-Capacitor.html

[Kokotoni Will]

I've got a new 47uf 400V electrolytic spare, would that make a suitable replacement?

[PJL]

My calculations say it is leaking about 3mA but it is possible it has not fully reformed yet. With V7 & V8 out, is the voltage on C23 increasing slowly? If not then replace it with your 47uF.

[Kokotoni Will]

https://youtu.be/xrKaHfShDnk?si=rONZkczapL8teg7k

It was increasing at about this speed, but it got stuck at 130V, and then started gradually decreasing. I'll have a look at changing it tomorrow.

[Kokotoni Will]

I've desoldered the + of C23, and soldered R39 to the new 47uf cap. I've soldered the - of the new cap to the - of the can. Please see photo.

New readings:
V7 anode 1 (pin 1, R38): 155
V7 anode 2 (pin 6, R39): 154
V8 anode (pin 7, R42): 33

V1 and V2 measure as before.
Voltage across both C23 and C11 is 192V.
I got the same level of crackle from both speakers when applying test probes.

I think the hum was due to some dry solder joints on the valve bases - I've reflowed them and the hum has gone.

[PJL]

More likely to be dirty valve pins which can be solved with a clean with Servisol.

Looks like we are done... Does your CD player have a volume control? If so you can wire it up but it would be a good idea to include a 0.1uF in series with the amp input to block any DC and you could try to include that in the amp.

Congratulations for sticking with it as so many people would have given up.

[Kokotoni Will]

Originally Posted by PJL

More likely to be dirty valve pins which can be solved with a clean with Servisol.

Looks like we are done... Does your CD player have a volume control? If so you can wire it up but it would be a good idea to include a 0.1uF in series with the amp input to block any DC and you could try to include that in the amp.

Congratulations for sticking with it as so many people would have given up.

Thanks a lot! Of course, I would still be at square one if it wasn't for the help of yourself and the many other kind souls of this forum!

As a test, I am currently listening to Hozan Yamamoto's "Silver World" via my Pro-Ject X2 turntable, and I have never heard an amplifier anything like it!

There is some background hum on both channels though. I'll add 0.1uf caps to the inputs tomorrow and see if that helps.

[Kokotoni Will]

I've started to build a passive preamp using the parts circled in red (see picture). The schematic is for a Mullard 5-10. Will this make a suitable preamp?

[PJL]

As the hum is on both channels it is quite probably an earth loop problem. I assume you are using another amplifier to connect to the turntable? Does it hum when the inputs to the valve amp are disconnected?

[Diabolical Artificer]

Well done, you drew a schematic that's commitment fair play to you.

Will this make a suitable preamp?

Because it's a passive circuit there will be attenuation, loss of volume but your amp AFAIK has high gain. Build it on a breadboard & give it a listen.

Andy.

[Kokotoni Will]

Originally Posted by PJL

As the hum is on both channels it is quite probably an earth loop problem. I assume you are using another amplifier to connect to the turntable? Does it hum when the inputs to the valve amp are disconnected?

I'm running my turntable through a Korg Kaoss Pad KP1, which I am using as a phono preamp, with the built-in input sensitivity pot working as a volume control. The hum does not occur when the inputs to the amp are disconnected.

I'm running a ground wire from the turntable to my living room radiator pipe, via the Kaoss Pad, which is necessary to remove loud humming.

[PJL]

The convention is to connect the turntable earth lead as close to the phono amp inputs as possible. Most phono amps will have a screw terminal to accommodate the earth wire but you could use one of the adjacent sockets to make your connection..

[Kokotoni Will]

I've built a passive volume control on a breadboard. With the signal going through that, there is now no hum.

I've not added any DC blocking capacitors yet. I have some 0.047uf film caps by various makes (WIMA, TCC, etc). Will they do?

[Radio Wrangler]

Originally Posted by Kokotoni Will

. Will they do?

Voltage:
They'll be blocking only a low voltage, all being well, but if there is a major fault you'd want it to have enough voltage rating to block the full HT voltage of the valve amplifier. This capacitor not only serves to protect the valve amp from any DC on the output of your CD plyer/RIAA stage or whatever, it also serves to protect the solid state stuff from any fault voltage from the valve circuitry.

Capacitance:
0.047 isn't a bad starting point. Too low a value will give you a bass-cut effect. It can be calculated if you know the output impedance of the driving circuit as well as the input impedance of the amplifier. The easy way out is to just try it. Bridge across the capacitor and can you hear any difference in the loudness of bass notes?

This is where you run into a psychological problem. If you know you've changed something it is very very difficult to avoid thinking you hear a difference. Worse, if you only think you've changed something, you can imagine you hear a difference. Just a slight change in overall level can seem to be all sorts of much more complicated effects. The human brain's ability to con itself is quite remarkable.

This is where engineers and scientists opt for some measurement instruments to get a dependably objective opinion! Some audiophiles believe nothing can better their ears and so get trapped in circles chasing improvements in imagined effects. A sort of Flying Dutchman curse, to never arrive at a finished conclusion.

Sometimes you do hear real differences, but it's nice to have non-partisan back-up.

Many years ago, amplifier designers had reached a plateau where they thought they knew it all, but some listeners said there was something wrong with many (close to most) designs. The measurement people pooh-pooed this.

It turned out that there was not only an oversight in the standard approach to amplifier design, but there was a matching oversight in the way amplifiers were measured. A perfect storm.

It took time for the situation to become recognised/admitted. As a consequence, it has entered self-perpetuating folklore that no measurement is ever a good guide, and that no transistor amplifier has ever sounded acceptable to anyone without poor hearing.

An impartial view of this is that things have been learned and that with care, good sounding amplifiers can be made based on all-valves, all-transistors or a mixture of both devices.

The impartial view also goes on to say that you have to be careful considering expressed opinions.... even your own! It's too easy to let your imagination lead you up an imaginary garden path. Countering imaginary effects is extremely difficult.

Chasing imaginary goals can be extremely expensive. There are also so many different opinions out there, many mutually-exclusive although this is often not noticed. Some of the pseudo-science trotted out as justification can be hilarious, so long as you're just an observer and don't have any skin in the game...

David

[(EC8010)]

Well done on getting an undocumented amplifier working.

The pictures you showed earlier in the thread with clear gettering at the top of the valves are a sign that the valves were too hot for a sustained period. As was mentioned earlier in the thread, screening cans easily cause this problem unless they are the IERC cans with liners that thermally coupled the envelope to the can. Moral: Never put screening cans on output valves.

Hum is always a problem on valve amplifiers, but it can be eliminated (well, brought down to -90dB relative to full power). As PJL suggests, it's most likely to be an earth loop problem.

Looking at the original construction, it's beautifully made. The transformers are Admiralty pattern, and with CV valves, it's quite likely that a naval base supplied the parts. Although beautifully made, it follows industrial or test equipment practise, rather than audio. Ideally, the output valves should have been at the ends of the chassis and the two input valves in the centre, allowing a 0V bus bar to pass down the centre of the amplifier, bonded to chassis near the input valves, and avoiding hum loops. If you still have hum due solely to the amplifier, a possible solution is to change all the 0V wiring to star earth. Find some solid core wire that looks similar to that already used and install it with right angle bends and it will look original (I really think any modifications should be in the spirit of the original construction). Be warned, it would be a lot of work. But it's such a pretty amplifier that it justifies careful work.

If you have an oscilloscope and square wave generator, the next step is to see if the feedback is correct. I'm guessing that the original constructor was not aware of the differences between output transformers and may not have realised that feedback and stability need to be tailored to the output transformer.

[Kokotoni Will]

Thanks, David and (EC8010).

After experimenting with 0.047, 0.056 and 0.1uf caps, it seems like 0.1uf gives the highest bass response.

The passive volume control I've built, using a 1M stereo logarithmic pot, detracts from the sound somewhat; it sounds to me like some high and low frequencies disappear. So, I've gone back to using my Kaoss Pad's phono preamp input sensitivity pot as a volume control. Would an active (valve?) preamp help with this?

I've uploaded a video demonstrating the hum that occurs when using my Pro-Ject X1 turntable with a Korg Kaoss pad KP1 as a preamp. At 0:38, I show how the level of hum increases when the preamp's input sensitivity is turned up:

https://youtu.be/8rDQGwEOaMw?si=0_JV8usua2KYhUQ0

Neatly rewiring all 0V connections to star earth does sound like a challenge. I'll look into it.

[(EC8010)]

1M is too high a value for a volume control because it causes too high an output resistance that interacts with loading capacitance to cause an HF roll-off; 100k is a much better value directly at the input of a valve amplifier. And if it needs to drive a bit of cable, then you need to go down to 10k.

If your hum is due to the RIAA stage, then no need to modify the amplifier. But if the amplifier hums with no input, then you may have to consider the challenge of star earthing. Sadly, it's not guaranteed to eliminate the hum; your hum could be magnetically induced. You really want to eliminate all other possibilities before taking such a radical step.

[Kokotoni Will]

Originally Posted by PJL

I see you have disconnected the earth and 1K resistor from the secondary which I missed from my instructions.

For push-pull transformers the correct measurement for turns ratio is primary 'anode to anode' to secondary. Attached is the A2134 datasheet which shows how the optimal load impedance varies from 3-7.5K dependant on operating conditions (particularly the HT) none of which match our example with UL taps and low anode current.

With 250V HT I would aim for about 6K. The required turns ratio for various speaker impedances is then:
4 ohms = square root(6000/4) = 39
8 ohms = square root(6000/8) = 27
16 ohms = square root(6000/16) = 19

Remove valves and disconnect HT. Confirm anodes are primary 3 and 4.
I am a little concerned about unwanted capacitive coupling in the transformer so put a 1K resistor load on the first secondary (AB). Then put your 6.3V on 3 and 4 of the primary and measure the AC voltage on the primary 34 then measure the first secondary (AB). Move the 1K resistor load to the other secondary (CD) then measure the AC voltage on the primary 34 (again) and measure the AC voltage on this.

I've finally got round to measuring the turns ratio voltages on the left transformer with my Fluke 25. Here they are:

L Primary 3-4: 6.52vac
Secondary A-B: 0.1489 - 0.1508vac

L Pri 3-4: 6.56 - 6.59vac
Secondary C-D: 0.1489 - 0.1526vac

I'm now using some new 6 ohm speakers.