Solderin' time!

EdGrip

Well, it works. I just had a little rock out with it - it's biased on the cold side because I can't find my probe and don't want to solder in 1ohm resistors when I know I've got one somewhere, and the heaters are still running hot and the HT is still running a bit low, but the amp works. I'm still not very happy with the transformer situation, but I'll think about that another time.

EdGrip

Took it for a bit of a jam today: The inaugural session in my mate's new log cabin. I left it set to 250v because I didn't know what the voltage situation was at his place (left my meter at home). Turned the amp on, and it warmed up in silence for 30 seconds to a minte, and then started making a low hum/oscillation. A low note, lower than the low C I was playing but higher than the fastest speed on the trem. It didn't seem like 50hz, but I could be wrong. It seemed to decrease when I put my hand firmly on the speaker grill, or firmly held the cab... Played some guitar through it, it was otherwise fine, but when you stopped playing the humming was still there. It got no louder or quieter. Still no sign of my nice new bias probe which hasn't been used yet...

Misterg

If you're bothered about the heater voltage, you could stick a 0.1 ohm / 5W resistor in each leg of the heater supply - they'll knock ~0.5V off the heater voltage without breaking into a sweat.

EdGrip

Cheers! That did occur to me. I may do that once I have let the amp settle in and am in possession of all the facts. It occurs to me to that I can now use a GZ34 to up the plate voltage, giving me a more vintage 5U4GB option and a cleaner, higher voltage GZ34 option, as opposed to before where the options were 5U4GB (nearly too many volts) and GZ34 (definitely too many volts).

So, I've got two brand new NOS 5U4GB - any of you lot fancy a swap for a good GZ34/5AR4? 

ICBM

Try a 5V4 - halfway between the 5U4 and the GZ34 in terms of voltage drop. (A little lower current-rated, but that won't matter here.) They're not expensive by NOS valve standards, and seem to have a particularly nice sound for some reason… at least in the amps I've tried them in.

But I would keep your 5U4s, at least until you've sorted all this out. You may actually prefer the tone with the lower voltage, and it certainly isn't going to do any harm.

EdGrip

I FOUND MY MOTHERFUCKING BIAS PROBE. 

Just wanted to spew my relief somewhere.

Misterg

EdGrip said:

I FOUND MY MOTHERFUCKING BIAS PROBE. 

Aliens must have borrowed it...

(You might want to give it a wipe)

EdGrip

I have had a thought about bringing down the voltage on the heater winding by increasing the current demand on it:

Bulbs.

I believe the standard Fender bulb is 0.15A. Keep a variety of others on hand to tweak the voltage down rather than adding resistors in series. 

If you think this is a good idea, I might buy some of the most useful current difference and sell them for the purpose.

EdGrip

Hmm. Occurs to me that doing so might stress the transformer more than is healthy.

Misterg

EdGrip said:

Hmm. Occurs to me that doing so might stress the transformer more than is healthy.

You could be right....

I did a rough reckoning, and think that you're probably drawing just over 2 amps from the heater  supply, I doubt that another 150 or 300 mA will make much difference to the voltage.

ICBM

You definitely want to do it by adding a bit of resistance in series, not by increasing the load on the PT winding.

EdGrip

I'll add it to the "Flawed Late-Night Schemes" folder.

EdGrip

Biased the amp this morning. With the big 5U4G, the plate voltage settled at about 340v. 

The oscillation is a problem still - I worked around it by piling pillows in front of the speaker, which eliminated the problem but as soon as you take them away.... 

.....bwwwwWWWWMMMMMMMMMMMMMMMMMMMMM..... put the pillows back, stops it dead immediately. It's some kind of microphonic feedback.

V1 was very audible if you tapped it, so I took it out and replaced it with another, still does it. Before I put the amp back together, is there anything besides microphonic valves which would cause this? 

hywelg

Suggest you put it on a dummy load and check its not acoustically coupled feedback. My hotplate is useful for this as it has a line out socket for monitoring.

EdGrip

Unfortunately, I've no access to such a thing. 

ICBM

EdGrip said:

Biased the amp this morning. With the big 5U4G, the plate voltage settled at about 340v. 

The oscillation is a problem still - I worked around it by piling pillows in front of the speaker, which eliminated the problem but as soon as you take them away.... 

.....bwwwwWWWWMMMMMMMMMMMMMMMMMMMMM..... put the pillows back, stops it dead immediately. It's some kind of microphonic feedback.

V1 was very audible if you tapped it, so I took it out and replaced it with another, still does it. Before I put the amp back together, is there anything besides microphonic valves which would cause this? 

Low-frequency microphonics are more often power valves than preamp.

Sometimes caps can be so too, although not usually enough to cause runaway feedback. If it's not the power valves, try tapping components in the signal path, that should tell you where it is.

EdGrip

One other thing - are you sure 5W is enough for the heater resistors that I may end up buying soon? Thanks again for your support, chaps. 

EdGrip

Ah, didn't see your reply, ICBM - thanks. I will investigate.

ICBM

EdGrip said:

One other thing - are you sure 5W is enough for the heater resistors that I may end up buying soon?

Yes, easily.

EdGrip

Thanks for that.

Now. Pin 7 of V2 connects to the previous stage (and the reverb) via a 3.3M resistor and 10pf capacitor in parallel. I was poking about in the amp with a stick to see if anything responded oddly, and I found that poking this capacitor, resistor, and the wire which connects them to pin 7, resulted in my poking/scraping being quite loudly amplified. No other components behaved this way. I'll investigate further today. Needless to say, removing V2 makes the problem go away. 

Someone on the Vintageamps forum suggested the NFB, and that I may have connected the new OT out of phase. 

When the amp came to me, someone previously had disabled the NFB entirely. I decided I would see what it was like with a bit of NFB in the mix, and used a (very old - 1940s) 5.something Kohm resistor that I foraged from a big bin full of obsolete electrical components at Bletchley Park, thinking that it would amuse me to have an amp at least partially capable of codebreaking if required. So there are a few things that could have gone wrong there. I will disconnect the NFB and see if that stops it.