Troubleshooting 5f6a build

[ragamuffin]

Last night I finished assembling my second amp build, a 5f6a head with a few mods from Rob Robinette's 5f6a mod page.

The pilot light turns on, all of the tubes glow, and I get SOME sound, but it's garbled/distorted and almost seems like there's a gating effect...

I took voltage readings on all the pins, and found a whopping 990+ ac volts on pins 2 and 8 of the rectifier, as well as several other odd readings. What would these readings indicate? Thanks!

Also, I can't seem to find my bias probe anywhere... I have a new one on it's way from Eurotubes. Until then I'll keep the bias set at a low setting.

[Stevem]

Can you post up several good gut shots of the amp, especially the power supply area and please post the info on the power transformer you are using.

Thanks.

[ragamuffin]

https://imgur.com/a/bycXgQ6 gut shot album

Power transformer is this one from Mojotone https://www.mojotone.com/Bassman-Pro-Su ... mer-125P5D

[sluckey]

found a whopping 990+ ac volts on pins 2 and 8 of the rectifier

Set your meter to measure DC VOLTS for pins 2 and 8 for the rectifier. No telling about the other weird readings. Most likely operator error or wiring error.

[ragamuffin]

found a whopping 990+ ac volts on pins 2 and 8 of the rectifier

Set your meter to measure DC VOLTS for pins 2 and 8 for the rectifier. No telling about the other weird readings. Most likely operator error or wiring error.

Pins 2 and 8 read 448 dc volts

[ragamuffin]

Ok so those "1.0?s" were an operator error, wrong voltage range. Looks like the majority of the voltages are good, with exception of V4 and V5 pins 4 and 6, which read very low.

Updated voltage readings:

[martin manning]

Are all three of these leads soldered to the eyelet where the 4700 ohm resistor is soldered?

[ragamuffin]

Are all three of these leads soldered to the eyelet where the 4700 ohm resistor is soldered?

Thanks! It appears that I missed that jumper connection; my jumper went straight from from the top of the board to the through hole. I had to undo some wiring to get at it and I haven't gotten to reassemble everything yet; I'll update when I do!

[ragamuffin]

So I added in the missing jumper under the board and put everything back together, and now things are even weirder unfortunately.

In standby I get proper voltage readings on the rectifier tube, but only AC heater voltages on the power and preamp sections.
When I switch out of standby to "on" the rectifier tube starts to redplate

Gut shots: https://imgur.com/a/bycXgQ6
Any ideas on what I could have done? Thanks

[martin manning]

In standby I get proper voltage readings on the rectifier tube, but only AC heater voltages on the power and preamp sections.

This is correct.

When I switch out of standby to "on" the rectifier tube starts to redplate

Can you dial the bias voltage all the way down and stop the red plating? You might need -50V or lower on pin 5’s. Do you have some way to measure power tube plate or cathode current?

[Stevem]

Martin, he is saying his Recto is red plating not a output tube.

[Stevem]

It’s kinda hard to tell from your photo, but it looks to me like you have the orange 600 volt ground / death cap connected to your standby switch and not the AC power switch.

[ragamuffin]

It’s kinda hard to tell from your photo, but it looks to me like you have the orange 600 volt ground / death cap connected to your standby switch and not the AC power switch.

Thanks, the orange death cap is wired from the ground lug to the upper left lug of the standby switch like this (from Mojotone schematic)

[ragamuffin]

I built a lightbulb limiter yesterday and and powered up the amp with the limiter this morning. When i put it in standby it briefly lights the bulb and slowly dims, and when I take it out of standby the bulb lights up and stays bright. This means that I have some kind of short, right?

[martin manning]

Or if not a direct short, a very high current draw.