71 Solarus external bias question

[Kevin Sparka]

Hi all, great site! I just picked up my first Sunn (Solarus) and am looking for some info that my search skills are failing me.

This amp appears to be 1971 based on markings and has 6550 power tubes, 5AR4 rectifier, 6AN8 between power tubes, and ECC82 & ECC83 preamp.

The amp has what appears to be two typical external bias features; a small red test point and a larger jack which I assume to be to adjust the bias pot via screwdriver. However, when looking inside the amp the test point is simply wired to the center lug of the pot, and the pot itself has another slot to adjust on the bottom. So, my questions are:

What reading would the test point be giving me based on that setup? And with that reading, what was Sunn intending the range to be if that was the only data presented without opening the amp?

What is the larger jack used for if the pot can be adjusted from inside the chassis? Is it dual sided adjust?

I am having issues posting with photos so trying this post without them for now.

Thanks for any/all help!

[loudthud]

It sounds like your amp has been modified. Please post some pictures.

In the early days, when consistent high quality 6550's were available, the bias pot was just adjusted to a Voltage, I think it was -55V. With today's high mains Voltage and tubes of unknown quality, it's best to use some means to measure cathode current and adjust bias for plate dissipation or crossover distortion.

[Kevin Sparka]

Thanks for the info thus far, still having issues both uploading as attachment and posting in message. Below are Imgur links in the meantime.

I have actually seen these on a few other amps in this forum so I am led to believe they are stock but it's also possible it was a popular mod? I know these "S" amps had a lot of running changes.

Top of chassis:
https://imgur.com/zGqEKhP

Pot inside with connection:
https://imgur.com/pRhIOsd

[Kevin Sparka]

Ugh, sorry about the pics...will keep playing with it.

As an update, I used a thinner screw driver and found the pot can indeed be adjusted thru that top jack so the pot must be double sided. I also went ahead and stuck a probe in the test point set for DC volts and found it to be -70.0 so you are correct that it pulls a negative voltage from the pot.

Obviously, I will need to test plate voltage etc as you suggested but out of curiosity I set it to -55v and didn't notice immediate red plating but put it back to -70 for now.

Appreciate the info! Looking to take it slow and learn as much as I can as I think this amp will be a lifelong keeper!

[Isaac]

My two cents:

Yes, the pot can be adjusted from the top or the bottom.

My older (60s) heads don't have the test jack, but my Model T (very early 2G, c. 75) does, so Sunn started adding that feature at some point in the 70s.

I don't think I'd go as far as -70V, but I don't see how it could hurt anything.

[tboy]

Images:

[loudthud]

I contacted tboy (system admin) and he fixed a folder size problem that was causing trouble with attachments.

What I like to do on these amps is install 1 Ohm resistors on the tube sockets. Move any wires on pin 8 to pin 1. Then install resistors between pin 8 and pin 1 so that the pin 8 side is up in the air where you can attach a test lead.

[Kevin Sparka]

Appreciate everyones help with getting the pics up and the info/suggestions.

A couple related questions to keep it all in one place if anyone is willing to chime in with their experience or knowledge:

Any idea what the original plate voltage was intended to be on these tube rectified 6550 Solarus back in the day? Regardless of that, what would be the ideal range it could be OK to run on a reading now?

Based on that plate voltage, would you still want to run under 70% of max plate dissipation like most other class A/B amps or is it different on these ultra linear designs?

Thanks,
Kevin

[Isaac]

With that, you've gone beyond my level of knowledge, Kevin!

[Kevin Sparka]

Hello all, finally got some time to test a few things out on this amp yesterday and have some findings as well as a question.

Plate voltage with the 5AR4 is 500 volts. I have looked at a lot of schematics out there for the KT88/6550 and while there are differences one consistent I keep seeing is the -55v on the bias pot setting. That being said, at -55 with this plate voltage it puts the tubes (EHX 6550) at over 85mA!

To get a bias around 70% I set the pot at -65v which gives me about 55mA

I also have a 5R4WY (the big potato masher) I put in as a test. With that, the plate voltage drops to 477 which coincidentally gave me just under 55mA when I set the bias to the -55v seen on all the schematics. This leads me to believe the plate voltage was intended to be in that ballpark with the lower wall voltage back then.

So, my question is?..has anyone seen issues in a Sunn amp running the 5R4WY rectifier (or have a reason why it is a terrible idea)? I am using it for guitar only and typically like a bit of sag so I am OK with that.

Thanks all,
Kevin