Bassman 59 LTD Power Cap Question

[Jazzcab]

Hello all!

Question for you guys...

I want to put a 5R4GB rectifier into my Bassman 59 LTD to lower the voltages closer to original specs and to facilitate some original 5881's.

In the schematic shown C16 and C17 are 100uf. Would I be better off swapping those out for 47uf's to accommodate the 5R4 (max capacitance 20uf)?

If so do I need to change up anything in regards to resistor values (R34, R35 etc) or the choke?

Thanks in advance!

[shooter]

C16 and C17 are 100uf. Would I be better off swapping those out for 47uf's to accommodate the 5R4 (max capacitance 20uf)?

worth a try, do it slow so you don't let Gremlins in, they like mods:), might wind up with some hum, but that's later 

[ac427v]

I have used a 5R4GYB with 22uf reservoir without problems. But was too chicken to push it more with 50uf (two 100uf in series). I see that the Bassman 59 LTD owner's manual suggests a 5AR4 or 5U4GYB. The 5U4GYB would give almost as much voltage drop as a 5R4. Reliable NOS and JJ are reasonably available.

[Jazzcab]

I dropped in two 47uf's replacing the 100uf's. As Shooter mentioned it seems like I do have a little more hum with single coils now. 

Interesting that Fender recommends a 5U4 as an alternative. That tap must be rated at at least 3A? I went with the 5R4 and I'm getting 433V on the plates.

Added a 56k resistor across R41 to get a decent bias range for the 5881's.

I've never biased vintage Tung-Sol 5881's before. What's a good range to get life out of them but not be too cold at 433 volts?

[66Strat]

Vintage Tung-Sol 5881 is rated at 23 watts maximum plate dissipation. At 433 volts, 32 ma to 37 ma would be in the 60% to 70% range of maximum dissipation.

[Jazzcab]

Vintage Tung-Sol 5881 is rated at 23 watts maximum plate dissipation. At 433 volts, 32 ma to 37 ma would be in the 60% to 70% range of maximum dissipation.

Thanks!

[Jazzcab]

I ended up putting the 100uf caps back in. I was reading others running a 5R4 in there without issues. On another DIY amp forum people were pushing 250uf plus caps with 5R4's without issues.

Apparently the inrush current is the issue and with a robust 5R4 it's not an issue? Someone recommended adding a small value resistor right after the 5R4 and before the first cap but I noticed that Fender puts a thermistor to limit inrush current in this amp. Probably why no one had failures with putting a 5R4 in these?

[66Strat]

I ended up putting the 100uf caps back in. I was reading others running a 5R4 in there without issues. On another DIY amp forum people were pushing 250uf plus caps with 5R4's without issues.

Apparently the inrush current is the issue and with a robust 5R4 it's not an issue? Someone recommended adding a small value resistor right after the 5R4 and before the first cap but I noticed that Fender puts a thermistor to limit inrush current in this amp. Probably why no one had failures with putting a 5R4 in these?

You may get by with it, if you avoid hot switching, eg. using the standby switch or turning the amp off and on again while the tubes are still hot. The thermistor is a heat dependent resistor. When it is cold, it has a relatively high value of resistance (~150 to 250 ohms). As it heats up, its resistance value drops to a negligible value (~1 or 2 ohms). When it is hot it is not effective in limiting current until it has cooled sufficiently (eg. room temperature to be safe). Peak inrush current limit for a 5R4 is ~3 amps. See note b in the linked datasheet and Rating Chart III. Rating Chart III provides guidance as to the minimum resistance needed in the rectifier AC plate supply to stay within the 3 amp limit.
https://frank.pocnet.net/sheets/049/5/5R4GYB.pdf

[Jazzcab]

I just added one of those potato masher JAN 5R4’s as backup. Almost as big as the amp. 

Thanks guys!

[rhemedy1]

Apparently the inrush current is the issue and with a robust 5R4 it's not an issue? Someone recommended adding a small value resistor right after the 5R4 and before the first cap but I noticed that Fender puts a thermistor to limit inrush current in this amp. Probably why no one had failures with putting a 5R4 in these?

You may get by with it, if you avoid hot switching, eg. using the standby switch or turning the amp off and on again while the tubes are still hot. The thermistor is a heat dependent resistor. When it is cold, it has a relatively high value of resistance (~150 to 250 ohms). As it heats up, its resistance value drops to a negligible value (~1 or 2 ohms). When it is hot it is not effective in limiting current until it has cooled sufficiently (eg. room temperature to be safe). Peak inrush current limit for a 5R4 is ~3 amps. See note b in the linked datasheet and Rating Chart III. Rating Chart III provides guidance as to the minimum resistance needed in the rectifier AC plate supply to stay within the 3 amp limit.
https://frank.pocnet.net/sheets/049/5/5R4GYB.pdf

Strangely enough - I have about 30 5R4WGBs and was going to use them in a modded 5E5 and am also tinkering with the idea of building a 'SKILTONE' 5E7. I had been using 5V4s in those but got 30 of these things dirt cheap before realising hmmm... we may have a problem here with regard to the 1st stage filter.  I've scratched my head for a while and surfed the web over the past couple of days...

As I am free to devise the PS from scratch what would you recommend in those scenarios 66Strat or any other electronics gurus?

[sluckey]

I've got a Raytheon potato masher in my Sunn Sceptre. I also have about a dozen of the "regular" looking 5R4s that I've tested in different amps over the years. Never had a problem.

[66Strat]

As I am free to devise the PS from scratch what would you recommend in those scenarios 66Strat or any other electronics gurus?

If it were my project, I would limit the first capacitor seen by the rectifier to 20uF, or add some resistance between the transformer secondaries and the rectifier plates. Hot switching is where problems are most likely to arise. So avoid turning the amp off and on without allowing the tubes sufficient time to completely cool to room temperature. In other words leave the amp on until you are done playing.

[rhemedy1]

Thanks for the comments... I used to put 10W 470Rs in front of the 5V4s as a safeguard... I just couldn't remember where in the circuit I'd put them... getting a bit doddery in my young (61) age...

But yeah... really don't need much more than that (20uF) in any case for either design...

Many thanks again...

R. - AKA - yankeerob

Have a great weekend!!! 

[traynor]

I've used 5R4 with 80uf for years, no problem.