'75 Pro Reverb bias problems AA1009

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JSett
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Re: '75 Pro Reverb bias problems AA1009

Post by JSett » Sun Jan 16, 2022 2:34 am

So, I went to bed early and finally managed to get a decent night's sleep. I think being over tired wasn't helping my brain analyze this properly.

I pulled everything off the power tube sockets, got rid of all the old solder and made sure I was starting fresh. Put in some proper grounds to the chassis from the cathode with a big iron (always satisfying). Tidied up the wiring a little, relocated the B+ supply wire to the correct spot, new grid resistors, new screen resistors (I only have 680ohm here for now so those will do, pretty sure that just results in a cooler bias) and sorted all the connections.

Went through as per øøøøøøø's recommendation with the meter checking for rouge grounds and continuity on all appropriate points. These all read as good so I powered up and took readings. Both grids are reading at a solid and steady -47mV, screens are getting a strong 450-ish volts.

It's on bench test for now, letting the tubes bed in and thoroughly warm up. Bias on the meter is currently sitting at about 26mA which is a little cool but not crazy low. The bias pot inside is just a bias balance pot so I've got those as level as possible.

I've plugged a guitar in for a sound check and it's making the correct noises, no weird hums yet. Will come back to it in an hour, take another round of readings then hopefully zip it back up!

A preemptive 'thank you' to all helpers here
Silly Rabbit, don't you know scooped mids are for kids?

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JSett
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Re: '75 Pro Reverb bias problems AA1009

Post by JSett » Sun Jan 16, 2022 4:26 am

Took it off bench test. Tubes are a bit mis-matched but I can swap in a matched pair when they arrive, get it closer.

Chassis back in the cab and fired up, every seems to be in order.

When the 480ohm 5w resistors arrive I'll put those in along with the fresh tubes so it's back to spec. Otherwise I think we're all good!
Silly Rabbit, don't you know scooped mids are for kids?

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øøøøøøø
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Re: '75 Pro Reverb bias problems AA1009

Post by øøøøøøø » Sun Jan 16, 2022 8:47 am

Yeah, nothing wrong with 680R for screen resistors.

Screens will just run a touch cooler, and the sound might change a tiny touch (probably negligible)

Congrats getting it sorted!!

Also for the record, there’s nothing wrong with adding 1 ohm bias test point resistors… they should just be properly grounded to chassis (and should be precision parts, 0.5% tolerance or ideally better, for precise measurement

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JSett
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Re: '75 Pro Reverb bias problems AA1009

Post by JSett » Sun Jan 16, 2022 9:48 am

øøøøøøø wrote:
Sun Jan 16, 2022 8:47 am
Yeah, nothing wrong with 680R for screen resistors.

Screens will just run a touch cooler, and the sound might change a tiny touch (probably negligible)

Congrats getting it sorted!!

Also for the record, there’s nothing wrong with adding 1 ohm bias test point resistors… they should just be properly grounded to chassis (and should be precision parts, 0.5% tolerance or ideally better, for precise measurement
Thanks! Yes, I've used those resistors before to take readings off of but, to be honest, I have a bias reader these days so don't need to worry about it so much. I only took them off so as to get it back to as pure a stock form as I could so there was nothing to question when going through everything and troubleshooting.

I've given it a good playthrough this afternoon and it sounds great. Let's hope it holds up a little better now. Every Silverface I've ever had has been an absolute workhorse so, theoretically, this should be no different.
Silly Rabbit, don't you know scooped mids are for kids?

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Re: '75 Pro Reverb bias problems AA1009

Post by øøøøøøø » Sun Jan 16, 2022 10:03 am

Nicely done!

If your tubes are mismatched, the bias balance pot should be able to sort you out.

You can also convert the bias balance to the earlier style bias voltage adjust (read: control over plate current/power dissipation) with only a few tweaks and a resistor or two.

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Re: '75 Pro Reverb bias problems AA1009

Post by JSett » Sun Jan 16, 2022 10:11 am

øøøøøøø wrote:
Sun Jan 16, 2022 10:03 am
Nicely done!

If your tubes are mismatched, the bias balance pot should be able to sort you out.

You can also convert the bias balance to the earlier style bias voltage adjust (read: control over plate current/power dissipation) with only a few tweaks and a resistor or two.

I've got them as close as I can. I think the one that was in V7 took a bit of a hammering with the wild voltages so within the scope of the bias balance I can only get them so far. They are 20+yr old Groove Tubes out of my HRD so were pretty worn already, so no big loss. The new pair should only take a couple of days anyway. Which reminds me I should get some to go into the Hot Rod as well.

If the amp ends up being a long-term resident then I'll probably do the Bias Adjust modification. I've looked on the diagrams for the older ones and you're right that there's not much to it. Probably less than a half hours messing about.
Silly Rabbit, don't you know scooped mids are for kids?

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