200S Sunn

[Valvehead]

ya...Ive never seen this. Im going to move those !

[Valvehead]

Ok boys.........FIXED !!!

It was the oddball ground scheme the previous guy decided was proper.......

I apologize... I have only worked on 1 or 2 Sunn amps in the past . I didn't know this was modified...only that it was partially re- capped.

THANK YOU for all your knowledge !

Amp is WAYYY quieter now ....no more loud hum/ buzz

[glebert]

Ok boys.........FIXED !!!

It was the oddball ground scheme the previous guy decided was proper.......

I apologize... I have only worked on 1 or 2 Sunn amps in the past . I didn't know this was modified...only that it was partially re- capped.

THANK YOU for all your knowledge !

Amp is WAYYY quieter now ....no more loud hum/ buzz

Way to go! If possible give some details about how you did the grounds, mostly did you use the chassis as ground or a ground bus or something else?

[Valvehead]

Glebert , off the top of my head - ac cord gnd+ fil CT I think to a spot near PT

filters + bias supply + cathode of 6550 to a spot.

PI and V1 grounds to a point near v1.

input jacks ground to chassis and to the point near v1

spkr jack grounds to chassis.

it might be wrong... but the amp is wayyyyy quieter!

[Valvehead]

Another option is to place zener diodes across the node B cap. Assuming a 450V cap in that position, I'd use a string totalling 440V. If the raw unloaded B+ is 560V, lets allow for mains variation and add another 10% margin, and calculate from 616VDC. That means we'd need to drop 176V across the choke and 15k 2W dropper resistor. The resistance of the choke should be negligible, most will be dropped across the 15k. That gives us 11.73mA and just over 2W dissipation. That ~12mA shouldn't be a problem for the choke. Might be advisable to change the 2W 15k resistor for a 3W one, but this would protect the preamp filter caps.
The 11.73mA at 440V means just over 5W dissipation in the zeners, so 3x150V 3W zeners in series should be just about perfect, and land close to 440V.
This method would avoid changing operating voltages like adding a bleeder across the node C filter cap would do.




Depends on the scope model, many don't attenuate DC in AC coupled mode as the 1Meg is AC coupled from the input, but other scopes DC couple the 1Meg even in AC coupled mode. Both my Tektronix 2465B and Rigol DS1052E attenuate DC when in AC coupled mode. This can be confirmed by switching into AC coupled mode and measuring resistance with a DMM connected to the input. Both my scopes read 1M when in AC coupled mode.
If a scope reads OL in AC coupled mode though, it will not attenuate DC in that mode. Very important to know your equipment well when doing high voltage measurements!

So , something like this ? If changing the 4 100uf caps to 47uf .
What about the 4 resistors across the 47uf's ...keep that value ?

[Greg Robinson]

So , something like this ? If changing the 4 100uf caps to 47uf .
What about the 4 resistors across the 47uf's ...keep that value ?

Zeners are backwards - they need to be reverse biased to get the zener/avalanche voltage - in forward biased mode they just act like regular silicon diodes with a ~0.7V drop.
You switched to solid state rectification from the tube rectifier right? So are those your current loaded node voltages? If so, you'd want to use 500V rated caps for the two 22uF's, and use a zener string totalling just under 500V. Say 3x 160V 5W zeners for 480V. This will avoid zener clamping the node during operation (with your recorded node voltage of 467V), but protect the caps during heater warm-up.
It's important that both 22uF are rated for 500V if you proceed in this manner, as node C will rise to the same voltage as node B before the heaters are warm.

220k 1W should be fine for the balancing resistors across the series caps if they're rated for 350V or greater.

[Valvehead]

Zeners are backwards - they need to be reverse biased to get the zener/avalanche voltage - in forward biased mode they just act like regular silicon diodes with a ~0.7V drop.
You switched to solid state rectification from the tube rectifier right? So are those your current loaded node voltages? If so, you'd want to use 500V rated caps for the two 22uF's, and use a zener string totalling just under 500V. Say 3x 160V 5W zeners for 480V. This will avoid zener clamping the node during operation (with your recorded node voltage of 467V), but protect the caps during heater warm-up.
It's important that both 22uF are rated for 500V if you proceed in this manner, as node C will rise to the same voltage as node B before the heaters are warm.

220k 1W should be fine for the balancing resistors across the series caps if they're rated for 350V or greater.

The owner took the amp today --i will have it back in 7days. the voltages in the drawing are with a Tube rectifier , and V1 pulled ( might have been v2) . Yes , the voltages went up when i put the SS in there . I would have to get the amp back to test the voltages with the SS rect. If thats necessary let me know asap and ill get the amp back .

right now it has the SS rect in it. He wants a tube back in there so i ordered a new Sovtek today and i want to make the changes to the filters / add zeners and see if the sovtek 5ar4 will be ok . I had 2 older/ used 5ar4 and they lit up like a Tesla coil with all the tubes installed .

yes , the 22uf are 500V

Thanks for your response!!

[Greg Robinson]

Ok, regardless of tube rectified or solid state, 500V rated caps for the 22uF would be better anyway, so that's good. And the higher zener voltage will mean less excess heat in unloaded conditions too, so that's also better.
Your voltage readings won't be accurate unless all tubes are fitted, so once you reinstall V1/2 they'll be a bit lower.

BTW, if you're going back to tube rectification, you should also add backup silicon diodes - the rectifier will have a longer life and if it DOES fail, the silicon diodes will protect the power transformer from damage.

[Valvehead]

Ok, regardless of tube rectified or solid state, 500V rated caps for the 22uF would be better anyway, so that's good. And the higher zener voltage will mean less excess heat in unloaded conditions too, so that's also better.
Your voltage readings won't be accurate unless all tubes are fitted, so once you reinstall V1/2 they'll be a bit lower.

BTW, if you're going back to tube rectification, you should also add backup silicon diodes - the rectifier will have a longer life and if it DOES fail, the silicon diodes will protect the power transformer from damage.

diodes on the 6550 plates ? Am i remembering right...1n4007 plate to gnd ?

[Greg Robinson]

Silicon diodes in series with the tube rectifier plates. 1N4007 does not have adequate voltage rating, as for full wave rectification the voltage rating must be for the full peak voltage of the whole HT winding (with allowance for unloaded voltages and wall voltage variation). Many/most would recommend using 2 or 3 1N4007's in series, but differences in reverse leakage and junction capacitance does not guarantee equal voltage sharing. Diodes with appropriate voltage ratings are easily and cheaply available, so that's my preference. 1.5kV diodes such as R1500/R1800/R2000, EM513/EM516/EM518, HV1.5/HV2, etc are all appropriate.

[g1]

diodes on the 6550 plates ? Am i remembering right...1n4007 plate to gnd ?

Those would be OT protection.
For back-up diodes on the rectifier tube, see attached. D1 and D2 between the PT and the rectifier tube.

edit: simul-post with #86

[Valvehead]

Thanks--ill post back next week !

[soundmasterg]

The owner took the amp today --i will have it back in 7days. the voltages in the drawing are with a Tube rectifier , and V1 pulled ( might have been v2) . Yes , the voltages went up when i put the SS in there . I would have to get the amp back to test the voltages with the SS rect. If thats necessary let me know asap and ill get the amp back .

right now it has the SS rect in it. He wants a tube back in there so i ordered a new Sovtek today and i want to make the changes to the filters / add zeners and see if the sovtek 5ar4 will be ok . I had 2 older/ used 5ar4 and they lit up like a Tesla coil with all the tubes installed .

yes , the 22uf are 500V

Thanks for your response!!

Here is some info I grabbed years ago off a post on one of the forums about the best quality modern GZ34/5AR4 to get. I believe it is probably better than the Sovtek/Tung-Sol/Mullard/EH/ etc.,

The Ruby 5AR4/GZ-34 is made in Tom's own factory as a unique part. We modeled it after one example of a new old Mullard tube he had and tried various versions of materials and spacing until it came very close, closer than any other makes of 5AR4 by the other major brands in the 50s and 60s. That Mullard tube really performed differently than others with the same designation, with lower forward drop, higher current etc, the more it was tested the more impressed I was with the original. If you have an amp that uses one, or are designing one, that is the only tube rectifier I would recommend for a guitar amp.. Smaller loose sounding harp or blues amps could use the 5U4 for a looser compression effect that would be nice. The higher impedance of that 5U4 means less voltage regulation so the difference in response and note recovery.

Greg

[Valvehead]

Zeners are backwards - they need to be reverse biased to get the zener/avalanche voltage - in forward biased mode they just act like regular silicon diodes with a ~0.7V drop.
You switched to solid state rectification from the tube rectifier right? So are those your current loaded node voltages? If so, you'd want to use 500V rated caps for the two 22uF's, and use a zener string totalling just under 500V. Say 3x 160V 5W zeners for 480V. This will avoid zener clamping the node during operation (with your recorded node voltage of 467V), but protect the caps during heater warm-up.
It's important that both 22uF are rated for 500V if you proceed in this manner, as node C will rise to the same voltage as node B before the heaters are warm.

220k 1W should be fine for the balancing resistors across the series caps if they're rated for 350V or greater.

So the zeners banded end goes to the B+ and not to ground ?

[The Dude]

Yes.