Throwing metal film resistors in Fender amps

[Dave H]

tubeswell, I don’t think it is the OT as it runs fine with JJs. The TS red plate at idle but you could short out the OT primary at idle (as is done for OT shunt bias measurement) and it wouldn’t make the tubes red plate. I’m starting to think it has to be the TS tubes. What else could it be?

Wil, What is the idle bias current of the TS tubes when they are red plating? Surely it can’t be only 27mA.

[tubeswell]

tubeswell, I don’t think it is the OT as it runs fine with JJs. ... I’m starting to think it has to be the TS tubes. What else could it be?

Yeah me too. I just stuck the OT post in so Wil could check it if he wanted to and put his mind at ease about that.

[frus]

Wil, What is the idle bias current of the TS tubes when they are red plating? Surely it can’t be only 27mA.

or if it is, they are definitely faulty

[Wil]

Hello Frus and tubeswell,
Thanks for the time you take to help me!

Did not have much time yesterday but I quickly try setting the bias as low as 13mA and this time (only let them warm for 3/4 minutes) they did not red plate.
An additionnal component that I "recycled" from the DRRI is the bias pot which has been trough a couple of soldering/desoldering. The negative voltages on the power tubes are fine but still I will replace it with a new one.
I believe or want to believe the OT is fine since it puts out good plate voltages.

On a an other topic, is it possible that Hoffman ab763 misses a capacitor in the vibrato section ? Shouldn't there be a 25uF cap across the 4.7k resistor that goes from the cathode to ground ?

[tubeswell]

On a an other topic, is it possible that Hoffman ab763 misses a capacitor in the vibrato section ? Shouldn't there be a 25uF cap across the 4.7k resistor that goes from the cathode to ground ?

You can have that Rk bypassed or not, depending on how much gain you want the LFO to put out. If you want more 'slam' in the trem, then add a Ck by all means

[kg]

Your tube sets that have red-plated may be damaged so they will keep red-plating at reduced bias. You would have to try a new set at lower bias to find out for sure.

this is an important note.

hot tubes outgas, and gas in the envelope will induce grid ionization current, making the grids go more +ve. runaway and/or bias instability is the end result. lower grid leaks will help reduce the symptoms but the tubes will never be the same again.

[Wil]

this is an important note.

hot tubes outgas, and gas in the envelope will induce grid ionization current, making the grids go more +ve. runaway and/or bias instability is the end result. lower grid leaks will help reduce the symptoms but the tubes will never be the same again.

Week-end update.
The JJ 6V6s are red plating too... 23mA only. I have two blue "clouds" in both tubes. PLate and grid voltages are within specs. What could cause red plating if not plate, grid voltages nor bias current ? I'll check if there's residual AC current across the filter caps but they're brand new TAD and F&T ones so I quite trust them.
Everything is brand new, except the transformers (which are Mercury Magnetics). The amp sounds good but I'm sure it can still be better.

+ the tremolo is not working. I added the 25uF across the 4.7k like in the 6G16 schematics but know I only have a ticking noise (I had nothing before).
New board or not, there's like a curse with that amp that kills the vibrato and the power tubes.

[Tom Phillips]

...The JJ 6V6s are red plating too...23mA only. I have two blue "clouds" in both tubes...

Just to make sure you know... Red plating and the blue glow are two different things. Are you assuming that they are directly related / always occur together?

[Wil]

Just to make sure you know... Red plating and the blue glow are two different things. Are you assuming that they are directly related / always occur together?

JJ do glow blue like on the picture you attached (which seems to be fine) but they run hot, burning hot.
But all voltages are fine... how come ?
Bias current is the only one (current) I measured and it's fine (22mA).

(the tremolo doesn't work).

I thought replacing the board would make my problems come to an end, haha. It's not really the case.
Could bad transformers or choke cause that ?

What are the other components that could be faulty except the reservoir filter caps, the choke, the output transformer, the power transformers, the two .1uF that goes to pin 6 and the grid stoppers?

To resolve list :
-Red plating tubes
-Fix tremolo

Another 'thanks' for your support !

[g1]

Please post the grid, plate, and screen voltages at the 6V6's, especially the grid when the red-plating is occurring.
Also post your method for measuring the current (22mA).

[Wil]

Please post the grid, plate, and screen voltages at the 6V6's, especially the grid when the red-plating is occurring.
Also post your method for measuring the current (22mA).

With 1 ohm from cathode to ground :
-21.1mA and 21.3mA
Pin 3 385V
Pin 4 386V (was 381V with 1k5 instead of 470ohm)
Pin 5 & 6 -21V

The first time I powered the amp up it was relatively quite and the hiss is equal to when it used to be a DRRI.
When I pass my hand over the reverb wires (amp open) the ground noise increases.

The tremolo is very weak and also works even with no footswitch plugged.
I want to wire the tremo and reverb footswitch as the DRRI, it's now operating like it did on the DRRI but I don't think my stereo footswitch jack wiring is good. (just took the Hoffman layout and made the two RCA jacks one stereo 6.35mm one).

[Dave H]

Week-end update.
The JJ 6V6s are red plating too...

Are you sure. Do they look like the picture below? They are only dissipating about 7.7W by your last measurements. My JJ are at 13W and they don’t red plate.

[Wil]

Are you sure. Do they look like the picture below? They are only dissipating about 7.7W by your last measurements. My JJ are at 13W and they don’t red plate.

[ATTACH=CONFIG]27818[/ATTACH]

Here's the actual picture of my TS 6V6
http://music-electronics-forum.com/a...07-unnamed.jpg

[tubeswell]

Here's the actual picture of my TS 6V6
http://music-electronics-forum.com/a...07-unnamed.jpg

Yes - that is red-plating.

The JJ6V6S is not a 'real' 6V6 tetrode. The JJ plates are much bigger (same size as the Russian 6P3S/6P3Se plates) giving them much greater plate-dissipation capability. The JJ6V6S is marketed as a 14W tube, but in reality it can probably easily handle 16W.

Whereas the humble little 6V6GT is no more than 12W. What's more, the screen support-rods etc on the New Sensor 6V6s are crappy little things, belying their all-round vulnerability. I would personally shy away from using RI Tung-Sols in a BF amp. (Some tube manufacturers of yesteryear used to make a 14W 6V6 - designated the 6V6GTA, and you may find those sturdier if you can find any).

[g1]

Have you posted a schematic for this amp? If so, I could not find it. What type of tremolo circuit is it? You have tremolo and bias issues which makes me suspicious of bias failure, perhaps due to a bias modulation type trem circuit.
Is the grid voltage remaining constant when the tubes are red plating (is it -21V with tubes out and also when tubes are in and redplating)?