Metropoulos Forum

Hey guys!

I was finally able to complete my 12xxx project but there are some things I have yet to deal with.

First off I'm posting my voltage chart. (measured without the power tubes)


Pin 1 Pin 2 Pin 3 Pin 4 Pin 5 Pin 6 Pin 7 Pin 8 Pin 9
V1 142 0 1 3.46 3.46 144 0 1.04 3.44
V2 158.9 0 1.12 3.46 3.46 301 159 160 3.44
V3 226 27.3 44.3 3.3 3.3 218 28.5 44.3 3.3
V4 GND 3.48 518 516 -38 516 3.47 NA NA
V5 GND 3.49 517 516 -38 516 3.47 NA NA
V6 GND 3.5 518 517 -38 516 3.49 NA NA
V7 GND 3.51 518 517 -38 517 3.5 NA NA

There are some values different to what there should be.

I measured 103.2 VAC at the end of the bias supply dropping resistor. (27K)

Anyway I put the power tubes in and tried to bias them. My plate voltage with them was 495V.

These are the results:
V4=68mV, V5=62mV, V6=0(?? - I tried to change the tube with the same results), V7=60mV (trimmer set to minimum)

My bias range measured without the power tubes on pin5 is from -38V at minimum to -48V at max.

So... there is something wrong and I'd like to find out what it is. No bias at V6 is strange although I was able to measure all the voltages in the chart with results above. The tube was also burning just like the others. Could it be the socket? And what about those high voltages on the other tubes? Shouldn't be there about 35mV?

Thanks for all your help!

Here are some photos.

Make sure the bias resistor on the v6 socket is properly grounded (bolt tightened, etc) or the tube is turned off. Try boosting the 27k resistor to 47k to lower the bias voltage. No power tube grid stoppers?

Ok, I'll check it and replace the resistor. Unfortunately I have to put a regular carbon film there instead of a Piher. As for the grid stoppers, I've got four 5k6s there.

You were right about the grounding. The 1ohm resistor on the V6 socket is dead. I'll buy a new one and go on with testing. Thanks for your help!

I just realized I can remove the resistor and try to bias the tubes with my bias probe. I hope it doesn't matter that one of the bias resistors is out and the rest stay in place..

So... I removed the resistor, put there a buss wire. All is well. As for the bias issue... I didn't realize there is a negative voltage so when I thought the pot was on minimum, the tubes were fed the most. Stupid. But now it's ok, I put there a nice set of old brown base Teslas and I'm just about to crank it with my Les Paul.

Ok, I've been enjoying my amp quite a bit with no issues until yesterday. I turned on the power switch, let the tubes warm up a bit, than hit the standby switch and started to play. All was well but then I heard not very loud crackling noise and noticed that V6 and V7 are glowing like crazy. I hit the standby switch and waited for a while. Then I turned it on again and everything was ok. I measured the bias after that and noticed that the V6 and V7 lost about 8mA. I don't want this to happen again - I'm lucky not having the head cab yet. Wonder what would happen if I didn't notice.

Anyway... any idea of what happened and what to do to prevent a similar accident in the future?

What you saw is called redplating. It does no good to let you amp warm up on standby. Standby only runs the tube heaters. Where you want to let it warm up, is when you turn it off standby. Let it warm up for 10 plus minutes before playing. during this time (especially on new builds) you should monitor the tube bias to watch for redplating. If one tube goes crazy, it will usually take its partner with it. A lot of us run closer to 400v, so redplating is less common.

Thanks for the reply! Well, this answer is a lot better than I expected. At least I know this is normal with new builds. But I'll have too look for some more Teslas to replace that pair. The amp sounds still pretty good but I guess it's not ok to have like 8mA difference on bias between those two pairs.

jimmyride wrote:Thanks for the reply! Well, this answer is a lot better than I expected. At least I know this is normal with new builds. But I'll have too look for some more Teslas to replace that pair. The amp sounds still pretty good but I guess it's not ok to have like 8mA difference on bias between those two pairs.

Why not?
Aas long as each half of the OT has the same draw and none of them are biased at over 70% at idle. I find that this sort of mismatch gives more 'colour' to the tone with more harmonics when you are giving it some stick.
BTW I really like those Teslas but they are getting as expensive as Mullards here in the UK

Well... they were made in my country so sometimes it's not as difficult here to get some for a decent price. The problem is they are usually sold with no guarantee so it's always a bit risky. If you want a pair or a quad I can try to look for them.

jimmyride wrote:Well... they were made in my country so sometimes it's not as difficult here to get some for a decent price. The problem is they are usually sold with no guarantee so it's always a bit risky. If you want a pair or a quad I can try to look for them.

Hmmm - might be interested (trade off of price vs risk of non-guarantee) if you find some NOS

A pair or a quad? I can only test them in my amp. I have the bias set for the pair measured 85 mA so I can easily recognize good/bad ones by comparison to these.

neikeel wrote:

jimmyride wrote:Thanks for the reply! Well, this answer is a lot better than I expected. At least I know this is normal with new builds. But I'll have too look for some more Teslas to replace that pair. The amp sounds still pretty good but I guess it's not ok to have like 8mA difference on bias between those two pairs.

Why not?
Aas long as each half of the OT has the same draw and none of them are biased at over 70% at idle. I find that this sort of mismatch gives more 'colour' to the tone with more harmonics when you are giving it some stick.
BTW I really like those Teslas but they are getting as expensive as Mullards here in the UK

Neil,

Are you arguing that matched tubes are not all they are cracked up to be? To a certain extent? I have thought for a while that back in the 60s and 70s when a tube was replaced they didn't do an entire matched set. You do get some "colour" that it hard to describe.

Thanks