59 Bassman Reissue LTD - Low Secondary voltages

[lowell]

The schematic calls for 491v on the OT center tap. I am only getting 440v in this amp. The current is only 20ma so it's not low voltage because of the power tubes drawing tons of current. Rectifier is a 5AR4. Brand new pair of JJ 6L6 installed.

Is there a different schematic for the LTD version?

I was curious if the schematic is accurate on that voltage spec or if anyone has any feedback on whether this is normal plate voltage or not. I discovered this because I felt the amp was distorting rather early and didn't have much clean headroom. Maybe it's normal, this is a desirable effect for a tweed amp.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Helmholtz]

The voltages in the BM RI schematic refer to the depicted SS rectifier. With the 5AR4 the voltages will be about 20V lower. Also if I read correctly all voltages refer to a mains voltage of 125V. Notes say voltages may vary +/-20%.
What is your actual mains and heater voltage? Remember 1V difference in mains voltage will change B+ by about 4V.

The original 5F6 schematic specifies a B+ of 432VDC (probably +/-20%).

[Pedro Vecino]

If your amp does not have an adjustable bias resistor it must be an old version (reissue, but old). With fifteen years or more probably. Itīs quite possible that it originally incorporated a pluggable solid state rectifier into the socket. I don't remember seeing 490V in them.
440V is an ideal voltage in a 56FA. Simply you would have to transform the bias circuit to obtain a higher current (32mA per tube for those 6L6, for example) and consider using a 12AY7 or at least a 5751 in the first socket to facilitate control over the volume and the transition to overdrive.

EDiT: We have written at the same time.

[Helmholtz]

BTW, the most critical and sound (and clean headroom) influencing tube in a BM (and amps using a similar circuit like many Marshalls) is the DC coupled cathode follower (V2). I recommend to try different tubes in this position.

[g1]

BTW, the most critical and sound (and clean headroom) influencing tube in a BM (and amps using a similar circuit like many Marshalls) is the DC coupled cathode follower (V2). I recommend to try different tubes in this position.

Unfortunately, you are sometimes limited to tubes that can survive in this spot, due to the heater-to-cathode voltage issue.

[Helmholtz]

Unfortunately, you are sometimes limited to tubes that can survive in this spot, due to the heater-to-cathode voltage issue.

IIRC from nickb's excellent analysis of DCCF failures, there was no evidence of heater-to-cathode voltage being the cause. Rather it seemed to be caused by high grid-to-cathode voltage and associated cold cathode current.

As this BM RI uses a (slow warm-up) 5AR4 rectifier, I don't expect the above effect(s) to be an issue here.

[g1]

Thanks. I didn't want to get to deep into the actual fault mechanism, just that it was the CF spot, but overlooked that the tube rectifier should mitigate the issue. Good to know.
I would guess though, that the majority of amps using CF follower would be SS rectified (you did reference 'many Marshalls'). So I would think that for most Marshall applications, you don't have as much to choose from for CF position? Have you had better luck with any type in particular?

[Helmholtz]

I would guess though, that the majority of amps using CF follower would be SS rectified (you did reference 'many Marshalls'). So I would think that for most Marshall applications, you don't have as much to choose from for CF position? Have you had better luck with any type in particular?

To answer your question: In my limited repair experience (mostly dating back to my shop days in the 80s - meanwhile I mainly take care of my own amp collection and projects) I never saw the kind of DCCF failure described by nickb with quality brand old stock tubes (Siemens, Valvo, Philips, Mullard, Brimar, Telefunken, RCA, Sylvania, GE) in a 60s, 70s and early 80s Marshall. I don't have much experience with newer Marshalls and tubes. If I had the problem I would think of a measure to limit grid-to-cathode voltage during turn-on, maybe using a small neon bulb (I have lots of them - would be a nice problem indicator, too).

But my comment above referred to sound influence. And tubes even of the same brand have considerable variation regarding grid conduction in a DCCF. So it makes sense to select the tube for sound (and separately take care of failure causes).