Rectifier plate resistance, Bassman issue

[Diverted]

Hi all,

Sorry for long post but I feel like I have to set the scene before I get to the point:
I recently gutted a 59 Bassman reissue and built a 5F6A circuit in its place. Used original reissue power transformer but switched OTs for one with a 4 ohm secondary and changed speakers for a 4 ohm load.

Anyway, the amp sounds great, voltages higher than vintage (467 B+ is about 35 higher) but I expected that looking at the reissue schematic. 6L6GCs are biased to 60 percent, drawing about 40ma each. Plates are 465, screens 463. Amp has consistently run at .6a at idle.

When I first built the amp we dropped in a modern 5AR4 and it flashed; I chalked it up to cheap modern Chinese stuff not being as rugged as the old 5AR4s. So we put in a Blackburn Mullard 5AR4. All has been good till then, until ...

Over the weekend he was playing and told me that after about two hours the amp started sounding "fuzzy" and he noticed a faint electrical smell. He shut it down and I picked it up.

Since then I have had it on my bench, playing guitar and music through it continually for hours at considerable volume and was finding no issues ... all voltages right at spec, amp drawing about .6 at idle. All was steady and the amp sounded great before I noticed something.

I was using an infrared heat gun to measure heat at various points due to the electrical smell comment. I noticed that one of the 5AR4 plates was running hotter than the other. Turned off all the lights and looked closely — that plate had a very small smear of red-plating going on. So I shut down the amp. Note I have been running series 1N5408 diodes to the rectifier plate leads. Also my power supply is not much different if at all from reissue: rectifier>first filter (50uf@700V)>choke>and then on.

Since noticing the red-plate I have been doing a deep dive and saw data sheets that show the minimum plate to plate resistance for a 5AR4 at 150 ohms. The actual resistance across the HV secondary is about 55 ohms. I have a bunch of questions about this:

1. This is the original power transformer from the Bassman reissue. That amp also ran a 5AR4 and didn't have issues. It also had a thermistor in the AC line, which I don't have. What allowed it to run at a transformer HV resistance that's a third of rated spec?
2. My thought is to add 50 to 100 resistors in series with the HV leads to the plates, to up the resistance the rectifier is seeing to anywhere from 150 to 250. If I do this, will this lessen the need for those series 1N5408 diodes to the plates? Or should I do series (HV lead>resistor>diode>plate)?
3. I'm wondering if I'm chasing a red herring. Does the HV leads' low resistance become less important after initial startup? In other words, is the GZ34 150 ohm rating more important during current inrush, than when the amp's been cooking for a few hours? And if that is the case, that brings up a ton of other questions.
Overall I feel very confident that the amp is built stoutly, 100 percent to stock with good parts etc and no bad solder joints. etc. It's possibly the lowest noise floor amp I've heard much less built, and all voltages and other characteristics have been spot on. If I should be looking elsewhere then I'm wondering my next step.

Any thoughts on the resistors and anything else that comes to mind would be welcome. Thank you!

[stratomaster]

Looks like a hot switched standby. 

I'd bet if your rewired that closer to say an AB763 Super/Deluxe Reverb, and put in a fresh rectifier tube that you'd be fine.

[Diverted]

Looks like a hot switched standby. 

I'd bet if your rewired that closer to say an AB763 Super/Deluxe Reverb, and put in a fresh rectifier tube that you'd be fine.

Thanks. Yes I see the first filter caps in the 5F6 are after the standby, not before like on the AB763. Thanks.
As for the HV secondary resistance of 50 ohms, would it be worthwhile to put a 5W 100 ohm resistor in series with each leg?

Only one more question regarding standby: The reissue also has that hot-switched standby shared with the original, which is as I built it. It makes me wonder why, if this is the issue, there weren't issues with it previously. Presence of the thermistor in the primary winding?

[stratomaster]

I don't see why you would need the series resistors.  Also make sure you're pulling HT from pin 8 of the rectifier. Looks like pin 2 to me, but photos and rotating sockets mentally while remembering pinout isn't 100% accurate for me right now.

Side note, what a weird bias arrangement.  The 22μFs and 270ks can all be replaced by a second 10μF @ 100v, and the 51k with the much more common 47k...

[acheld]

From your description it sounds like you are within nominal operating conditions.  But  . . .  I hadn't looked at the data sheet and the limiting resistors in a long while --  I don't normally use them.  Maybe they would help . . .  dunno.

I'd leave the diodes in series with the resistors.   

The input thermistor deals with startup current -- once it is warmed up it assumes a low resistance and stays there.

Interesting problem. 

[Diverted]

I don't see why you would need the series resistors.  Also make sure you're pulling HT from pin 8 of the rectifier. Looks like pin 2 to me, but photos and rotating sockets mentally while remembering pinout isn't 100% accurate for me right now.

Side note, what a weird bias arrangement.  The 22μFs and 270ks can all be replaced by a second 10μF @ 100v, and the 51k with the much more common 47k...

Thanks. I'd read somewhere long ago that either pin 8 or 2 was fine. If best practice is 8, thank you for correcting me and I will swap that lead out. As for the bias supply, yes strange but it is just the original 5F6a supply with a pot wired in for adjustability. It works as it should so I may just leave.
I appreciate the help.

[Diverted]

I'd leave the diodes in series with the resistors.   

Thanks, yes it is a different problem and not something I've encountered before. Quick question: Resistors prior to diodes, correct?

Ted

[stratomaster]

I don't see why you would need the series resistors.  Also make sure you're pulling HT from pin 8 of the rectifier. Looks like pin 2 to me, but photos and rotating sockets mentally while remembering pinout isn't 100% accurate for me right now.

Side note, what a weird bias arrangement.  The 22μFs and 270ks can all be replaced by a second 10μF @ 100v, and the 51k with the much more common 47k...

Thanks. I'd read somewhere long ago that either pin 8 or 2 was fine. If best practice is 8, thank you for correcting me and I will swap that lead out. As for the bias supply, yes strange but it is just the original 5F6a supply with a pot wired in for adjustability. It works as it should so I may just leave.
I appreciate the help.

Pin 2 is just for the filament. Pulling HT from there is a recipe for killing rectifier tubes.  If this started after your rebuild then that's the smoking gun.

As for the bias, I commenting on the stock Fender schematic.

[Diverted]

Pin 2 is just for the filament. Pulling HT from there is a recipe for killing rectifier tubes.  If this started after your rebuild then that's the smoking gun.
[/quote]

VERY good to know. Thank you.

[Diverted]

Just swapped out B+ lead from 2 to 8 and put in a pair of 100/5w Ohmite wirewound resistors. Wow, I didn't think they would drop B+ 20 volts!
Thanks again for the help.

[astronomicum]

Pin 2 is just for the filament. Pulling HT from there is a recipe for killing rectifier tubes.

I have read through these specs for the 5R4 rectifier and can find no reference as to which pin the HT is to be connected. I have read that the HT is to be connected directly to the cathode but neither spec indicates which pin that is. If pin 8 is connected directly to the cathode, where is that information coming from? What am I missing? Thanks.

[SEL49]

I have read through these specs for the 5R4 rectifier and can find no reference as to which pin the HT is to be connected.

Wrong tube. They are discussing a 5AR4.

The 5R4 is a directly heated cathode tube. The filament has a special coating which acts like a cathode. IOW, the filament is also the cathode. There is no separate cathode element. You can get the B+ from pin 2 or pin 8. Same applies to a real 5Y3 or 5U4.

The 5AR4 is an indirectly heated cathode. The separate cathode element is internally connected to the filament at pin 8.

[astronomicum]

Got it. Thank you.

[Merlin]

data sheets that show the minimum plate to plate resistance for a 5AR4 at 150 ohms. The actual resistance across the HV secondary is about 55 ohms.

There is also the primary resistance which is reflected across to the secondary (it will appear larger), so the total secondary resistance is more than just 55 ohms so you're probably fine. As Stratomaster said, fix the hot switching standby.

[pdf64]

data sheets that show the minimum plate to plate resistance for a 5AR4 at 150 ohms. The actual resistance across the HV secondary is about 55 ohms.

There is also the primary resistance which is reflected across to the secondary (it will appear larger), so the total secondary resistance is more than just 55 ohms so you're probably fine...

Certainly the reflected primary resistance will help to increase the effective anode resistance, but it seems unrealistic to expect it to increase it from under 30 ohms to nearly 75 ohms?