New Rectifier Tubes Pulling massive current.

[Intergalactic Tourister]

Hi,

I have 3 brand new rect tubes (JJ 5u4GB, EH 5u4GB, and JJ 5ar4) that ALL pull upwards of 100ma plate current on both brand new JJ 6L6GC tubes. Actually it maxes out my bias meter so it may go even higher... but I have to turn it off immediately.

However, when the original 1950s/60s Tung Sol 5u4GB is installed it pulls around 50ma @ 333v plate on both 6L6GC.

Bias pot sweep is smooth and functional and measures correctly throughout the sweep. Its currently set at coldest setting for both old and new rect tubes when turning amp on. All components test good.

Amp is 2x6L6, 2x12ax7, 1x5ar4, Fender-like circuit. 1 vol/ 1 tone. 22uf reservoir cap.

*With new 5u4GB and with 6L6s plugged in I get 339 B+.
*With new 5u4GB and without 6L6s plugged in I get 533v B+.

This is as far as I got for now. Any ideas?

Thanks ya'll!

[pdf64]

How about if you increase the magnitude of the bias voltage (ie the negative voltage applied to the 6L6 control grids)?
As perhaps the current 'cold' setting isn't cold enough.

Note that Fender have made a wide variety of amps, it would be helpful to identify the schematic that best resembles yours, with particular regard to its power amp, HT and bias supplies; perhaps include links to the transformers used.

Consider the possibility that the probe or meter are giving bad readings, and that the amp may be oscillating under certain conditions (eg with the change in operating conditions provided by new rectifier). Oscillation can be stopped by removing the phase splitter tube, so perhaps try that and re-check.

Are you sure that the amp has appropriate fusing? ie as a minimum a T rated fuse in the PT primary circuit, probably T2A to T3A, preferably with fusing on the secondaries too.

[stokes]

Cant say for sure what is going on with those current production rectifier tubes,but I dont use them because they arent as reliable as the old stock tubes.Back in the 90's there were some "5ar4's" coming out of China that were just relabeled 5Y3's.Old stock rectifiers are still available and well worth the extra $$$.Even used ebay tubes are more reliable than the current productions and pretty inexpensive.I would stick with the old stock Tung sol you have there.

[pdf64]

...I would stick with the old stock Tung sol you have there.

I'm thinking that one of the old rectifier's diodes may not be conducting, resulting in it only providing half wave rectification and a heap of sag.

[mozz]

Yes, sounds like the original tube could be bad, you may have other problems.

[Enzo]

Easy enough, is your ripple 60Hz or 120Hz?

[Intergalactic Tourister]

How about if you increase the magnitude of the bias voltage (ie the negative voltage applied to the 6L6 control grids)?
As perhaps the current 'cold' setting isn't cold enough.

Note that Fender have made a wide variety of amps, it would be helpful to identify the schematic that best resembles yours, with particular regard to its power amp, HT and bias supplies; perhaps include links to the transformers used.

Consider the possibility that the probe or meter are giving bad readings, and that the amp may be oscillating under certain conditions (eg with the change in operating conditions provided by new rectifier). Oscillation can be stopped by removing the phase splitter tube, so perhaps try that and re-check.

Are you sure that the amp has appropriate fusing? ie as a minimum a T rated fuse in the PT primary circuit, probably T2A to T3A, preferably with fusing on the secondaries too.

First thing I thought to do but the Tung Sol would operate at around 0ma in order to get the other rect tubes near 50ma.

Its close enough to an AB165. No name or #s on the amp but its from the 50s/60s. Xformers are old, no #'s as they have been repainted black some time ago (not by me).

Probe and meter work fine in other amps and components. Removed phase splitter. Still pulling too much current.

Has a mains 250v 2A slo blo only.

[g1]

Unless you have verified your original rectifier in another amp, you can't use it as a reference for normal current 'pull'.
330V plate sounds abnormally low for a Fender 6L6 amp to me.

[Intergalactic Tourister]

Easy enough, is your ripple 60Hz or 120Hz?

With the Tung Sol minimal 60Hz. I can't even turn it on for more than a second to hear what the new rect tubes are doing... as I get 100ma almost immediately.

I have a 22K bias pot w/ 15k resistor off the leg. My initial thought is to add something like 30K or so to see if the new rect tubes can bring the 6L6 into ballpark or near 50ma. That would mean the Tung Sol will be nearing 0ma. Could it be pdf64 is correct? Maybe the one of the diodes is not conducting in the Tung Sol.

[Intergalactic Tourister]

Unless you have verified your original rectifier in another amp, you can't use it as a reference for normal current 'pull'.
330V plate sounds abnormally low for a Fender 6L6 amp to me.

I thought about it. But wanted to get more info before I put the Tung Sol in another amp. Just in case it decides to short or something.

[pdf64]

...the Tung Sol would operate at around 0ma in order to get the other rect tubes near 50ma...

Sorry, I assumed that this was a new build project, rather than an old amp.
Maybe that rectifier is only working on one side AND the bias supply reservoir cap is bad (thereby hamstringing the bias supply); are the ecaps original?

Whatever, I think you are putting the cart before the horse.

Have you got a scope (or a meter that can read Hz, and Vac when superimposed on Vdc), if so then best to check the bias and HT ripple, as Enzo suggests.

[Intergalactic Tourister]

Hi,

I have 3 brand new rect tubes (JJ 5u4GB, EH 5u4GB, and JJ 5ar4) that ALL pull upwards of 100ma plate current on both brand new JJ 6L6GC tubes. Actually it maxes out my bias meter so it may go even higher... but I have to turn it off immediately.

However, when the original 1950s/60s Tung Sol 5u4GB is installed it pulls around 50ma @ 333v plate on both 6L6GC.

Bias pot sweep is smooth and functional and measures correctly throughout the sweep. Its currently set at coldest setting for both old and new rect tubes when turning amp on. All components test good.

Amp is 2x6L6, 2x12ax7, 1x5ar4, Fender-like circuit. 1 vol/ 1 tone. 22uf reservoir cap.

*With new 5u4GB and with 6L6s plugged in I get 339 B+.
*With new 5u4GB and without 6L6s plugged in I get 533v B+.

This is as far as I got for now. Any ideas?

Thanks ya'll!

EDIT EDIT EDIT: I mistyped.

*With OLD Tung Sol 5u4GB and with 6L6s plugged in I get 339 B+.
*With new 5u4GB and without 6L6s plugged in I get 533v B+.

Also, with new 5u4GB and with 6L6s plugged in I can't get a B+ reading as the current spikes to fast and too high.

[frus]

Sounds to me something's off with (one of?) your 6L6s, not the rectifier

[Intergalactic Tourister]

Sorry, I assumed that this was a new build project, rather than an old amp.
Maybe that rectifier is only working on one side AND the bias supply reservoir cap is bad (thereby hamstringing the bias supply); are the ecaps original?

Whatever, I think you are putting the cart before the horse.

Have you got a scope (or a meter that can read Hz, and Vac when superimposed on Vdc), if so then best to check the bias and HT ripple, as Enzo suggests.

Bias caps/e-caps are all new. Resistors all check good. Reservoir cap is new and tests good.

I dont have a scope.

[pdf64]

Can your meter measure Hz / Vac on the HT?

With no power tubes in place and the bias control set to 'as cool as possible', what are the Vdc on power tube socket terminal #s 3, 4 and 5?