Plate voltage drop in a Super Lead

[Afterall]

Can someone help explain this? First time replacing tubes and biasing a 1959 reissue:

I replaced the preamp and El34 tubes following videos on YT, I measured voltage at Pin 3 of 440v after a 10 minute warm up. The bias calculation would then be 39.7ma (bias seemed to be originally set at 25ma) but the bias pot maxed out at 39ma using a bias tester connected to a multimeter. I again checked the voltage and now get a reading of 140v at Pin 3.

A few questions:
1. Is the initial voltage of 440 low everywhere I look says it’s usually around 470v?
2. Is this possible for the voltage to drop so low from 440 to 140?
3. Should the bias pot max out so soon?

 

[AndyD]

I would say the initial voltage looks low to me. Indeed 470v is more typical. The massive voltage drop is concerning. You must have a short somewhere is my guess.

 

[Pete Farrington]

Measure the HT voltage, eg at the reservoir cap or OT CT.
The difference between that and anode voltage will be negligible.
If you must measure anode voltage, first remove the valve in V3.
Otherwise oscillation is easily induced, meaning that any reading obtained is invalid (as the amp is no longer at idle).

 

[Afterall]

Hi Pete, your explanation is a bit technical for me.
How do I go about measuring HT voltage?
What do you mean by “if you must measure anode voltage”?

 

[mAx___]

You can measure HT voltage i.e. at the HT fuse, etc. No need to measure it at the plate (anode). But if you prefer doing that, first remove V3. I think that's what Pete was suggesting.

 

[Afterall]

Ok, so basically how I originally measured at pin 3 only this time with v3 (PI) removed?
What should this Result then tell me?

 

[Pete Farrington]

How do I go about measuring HT voltage?

Set the meter to measure a high DC voltage. Touch the red probe tip to either of the HT fuse holder terminals, and the black probe to the chassis metalwork.

What do you mean by “if you must measure anode voltage”?

There are countless sources insisting that anode voltage must be measured.
It’s hard for me to appear credible arguing against that.
Hence I put forward a way of measuring anode voltage whilst mitigating for it causing oscillation. Thereby enabling valid measurements to be made.

 

[Afterall]

Ok, so I got back into the amp tonight. I got 150v at the HT fuse, I also pulled v3 and got 150v at pin 3.
I also pulled the HT fuse and tested, it has no continuity or resistance.
What does this mean?

 

[john l]

So I may be missing something but what era is this superlead? Ive seen multiple rocker switch era amps from the late 70s with a B+ of 450v. Same with 2203s as they used the same PTs at that time.

 

[Afterall]

It’s a 2012 w/effects loop. No mods, had the original tubes in it.

 

[john l]

Ok, so I got back into the amp tonight. I got 150v at the HT fuse, I also pulled v3 and got 150v at pin 3.
I also pulled the HT fuse and tested, it has no continuity or resistance.
What does this mean?

Having continuity implies no resistance. If youre HT fuse was blown the tubes the power indicator and tube filaments would light up but the amp would not produce sound. If the Mains fuse blows the amp will be completely dead.

It’s a 2012 w/effects loop. No mods, had the original tubes in it.

That could very well be totally normal for a reissue. Maybe someone here would be good enough to verify if they own one.

 

[Afterall]

Correct tubes light up. The fuse is blown, the filament inside is visibly broken, but I all did was just change the tubes.

 

[ReWind James]

You had the bias pot maxed in some direction between your normal HT reading with a working fuse and your unusual HT reading and a blown fuse. Are you sure you are measuring and setting the bias correctly? Logic would point to the problem happening between the two readings and while you were messing around with it.

What prompted you to change all the tubes (preamp / PI, included), to begin with? Was there already a problem with the amp?

 

[Afterall]

When i began I was getting a bias reading of about 25ma, the calc I did was supposed to be 39ma but I couldn’t turn the pot any more to reach the full 39ma. I was using a bias tester that you connect to a multimeter. I decide to check the reading on the last tube and then got a 0 reading, checked the plate voltage again and then got a reading of 140v. Thought I would have noticed something when the fuse blew.
Here’s the history:
I recently bought the amp off eBay. I found v1 to be microphonic, (maybe from shipping) swapped it out with a tube from another amp, then everything was fine (loud as hell) so I used attenuator. Started to notice a change in tone kind of thin and flat especially around the G string. So as I said it had the original tubes in it and the tube I swapped in was also old. So, I figured new tubes.

 

[neikeel]

To go from 450 to 150vdc is very strange.
I confess I hook ground to common point and probe pin3, can’t recall an issue although of course I accept Pete’s premise. I thought you might have a dodgy meter.
When biased hot vs cold you might have 50v difference in anode voltage but not 300.
Sounds like you need to record all the pin voltages hot and cold and report back. Check your meter leads and battery first (I know voltage measurement is passive but humour me for now!)

 

[Spanngitter]

I had 2 cases in the past where I had suspicious low B+ and both of these cases had been cause by the Standby Switch developing noticable resistance.
However, both units had been stored in a quite damp area (Basement with natural floor) and showed also lots of corrosion at the GND Connections due to the black oxide washers rotting away...

 

[Afterall]

To go from 450 to 150vdc is very strange.
I confess I hook ground to common point and probe pin3, can’t recall an issue although of course I accept Pete’s premise. I thought you might have a dodgy meter.
When biased hot vs cold you might have 50v difference in anode voltage but not 300.
Sounds like you need to record all the pin voltages hot and cold and report back. Check your meter leads and battery first (I know voltage measurement is passive but humour me for now!)

I have two meters and get then same reading. Am I measuring 150v now because the fuse is blown?

 

[Pete Farrington]

That seems a reasonable conclusion

 

[Afterall]

Maybe one of the new tubes was bad? Got to get some fuses.
Why was I not able to reach the full ma range on the bias pot?

 

[Pete Farrington]

I was using a bias tester that you connect to a multimeter.

Are you sure you had the meter set up correctly?
Inexperienced users often get it wrong.
Different bias probes are intended for different meter settings.
Most need the meter setting to measure milli volts, as they convert the current to an equivalent voltage.
A few read current directly and so need it setting to measure milli amps.