SRV in the studio with his SSS

[martin manning]

4x 6550 at 500V Va and a 2k load is right around 200W on paper. The screens would want to be about 325V. If you we're to take Va up to 550, screens to 350, theoretical power output would increase to 243W. That would probably yield something over 200W actual.

[Roe]

4x 6550 at 500V Va and a 2k load is right around 200W on paper. The screens would want to be about 325V. If you we're to take Va up to 550, screens to 350, theoretical power output would increase to 243W. That would probably yield something over 200W actual.

thanks, that would support the assumption of a 4.3k primary (a431 OT), wouldn't it?

[rogb]

The solo from that Zucchero session at 3:56 - this is great stuff!

[xtian]

So weird to see SRV without a hat!

[Matt J]

So weird to see SRV without a hat!

I was thinking the same thing!

Though, standing that close to his SSS at full bore might have blown his hat off!

[martin manning]

4x 6550 at 500V Va and a 2k load is right around 200W on paper. The screens would want to be about 325V. If you we're to take Va up to 550, screens to 350, theoretical power output would increase to 243W. That would probably yield something over 200W actual.

thanks, that would support the assumption of a 4.3k primary (a431 OT), wouldn't it?

4K3 doesn't make sense to me for 4x 6550, has to be around 2k. AFAIK the A431 OT is intended to deliver 60W with 2x 6550. See: http://triodeelectronics.com/a4dyoutr43oh.html Unless there is a screen supply around 2/3 of Va available, UL is the way to go. Either way, 450V Va would make 160W theoretical output power.

[talbany]

This is one of those questions nobody seems to know?. How did Dumble run the screen supply's on his amps loaded with 6550's running at 500V+. Some have speculated UL taps others say he didn't use any taps?
The problem is if you run the screens too close to the plate supply, the load line will be in the wrong place which leads to excessive screen current at higher power levels. This is why the transformers in the early Marshall Major/Plexi 6550 conversions needed to be upgraded to an OT with UL taps. So how did Dumble generate the lower screen voltages needed to run the 6550's safety? or did he ignore the load lines?

Anyone?

Tony

[Roe]

This is one of those questions nobody seems to know?. How did Dumble run the screen supply's on his amps loaded with 6550's running at 500V+. Some have speculated UL taps others say he didn't use any taps?
The problem is if you run the screens too close to the plate supply, the load line will be in the wrong place which leads to excessive screen current at higher power levels. This is why the transformers in the early Marshall Major/Plexi 6550 conversions needed to be upgraded to an OT with UL taps. So how did Dumble generate the lower screen voltages needed to run the 6550's safety? or did he ignore the load lines?

Anyone?

Tony

Bigger screen resistors would work, wouldn't it? At least 1k is necessary at 500v

[martin manning]

Dropping 150 or so volts with a resistor will make a very high impedance (and saggy) screen supply.

I see three options for getting Vg2:

1) Separate screen winding, ala SVT.
- This requires an unusual or additional PT.

2) UL ala Marshall Major and Sunn.
- This is the easiest option to implement.

3) FWB with CT to get ~1/2 of Va for Vg2.
- This requires an odd OT impedance half way between standard PP and UL, 3k in this case, and wastes a lot of the power capability of the 6550's.

HAD was certainly a student of what other amp builders were doing at the time, and adapted and improved upon their ideas for his designs. He also used available production parts, often from other MI amps. I'm inclined to think he went UL, since obtaining suitable transformers would not be a problem- the power stage is after all a Hi-Fi amp. Member dreric's recent UL SSS worked out quite well.

[Roe]

Dropping 150 or so volts with a resistor will make a very high impedance (and saggy) screen supply.

I see three options for getting Vg2:

1) Separate screen winding, ala SVT.
- This requires an unusual or additional PT.

2) UL ala Marshall Major and Sunn.
- This is the easiest option to implement.

3) FWB with CT to get ~1/2 of Va for Vg2.
- This requires an odd OT impedance half way between standard PP and UL, 3k in this case, and wastes a lot of the power capability of the 6550's.

HAD was certainly a student of what other amp builders were doing at the time, and adapted and improved upon their ideas for his designs. He also used available production parts, often from other MI amps. I'm inclined to think he went UL, since obtaining suitable transformers would not be a problem- the power stage is after all a Hi-Fi amp. Member dreric's recent UL SSS worked out quite well.

UL will work, but dumble used chokes (cf. https://ampgarage.com/forum/viewtopic.php?t=12058). Dumbleland #009 seems to use the Triad R-25A (400VAC, 500mA). Many different ampbuilders have used 500+ plate voltages and 1k-2k2 screen resistors. This tends to work, although not all tubes last long

[talbany]

Dropping 150 or so volts with a resistor will make a very high impedance (and saggy) screen supply.

I see three options for getting Vg2:

1) Separate screen winding, ala SVT.
- This requires an unusual or additional PT.

2) UL ala Marshall Major and Sunn.
- This is the easiest option to implement.

3) FWB with CT to get ~1/2 of Va for Vg2.
- This requires an odd OT impedance half way between standard PP and UL, 3k in this case, and wastes a lot of the power capability of the 6550's.

HAD was certainly a student of what other amp builders were doing at the time, and adapted and improved upon their ideas for his designs. He also used available production parts, often from other MI amps. I'm inclined to think he went UL, since obtaining suitable transformers would not be a problem- the power stage is after all a Hi-Fi amp. Member dreric's recent UL SSS worked out quite well.

UL will work, but dumble used chokes (cf. https://ampgarage.com/forum/viewtopic.php?t=12058). Dumbleland #009 seems to use the Triad R-25A (400VAC, 500mA). Many different ampbuilders have used 500+ plate voltages and 1k-2k2 screen resistors. This tends to work, although not all tubes last long

If your going to go that route your better off going with a KT-88 since their screens are rated at 8w vs the 6550 @ 6w. I've run 88's in an amp with 510V and 2K screen resistors in a modified Traynor for years using JJ"s. with no problem. Marshall ran 6550's in the later 80's JCM amps although they used a stout tube (Stock GE's) @ 465V. You can get away with it just be aware if you push them too far you can take out the screens or worse.
BTW. I know some Winterland amps had UL taps but I have never heard of an SSS or a Dumbleland running ultra-linear yet. Doesn't mean they dont exist. Just never heard of it??
Stevie was also known to have all kinds of maintenance issues with his SSS's on the road. it's no wonder the way he played and ran them hard
Hammond used an OC3 regulator tube to drop 100V to the screens to solve the issue. With the OC3 your not only dropping the volts you also get a regulated screen supply and it's cheap to boot

oc3 regulator.gif

Tony

[Guy77]

I have a friend of a friend that had Two Rock build them a Custom version of there Custom Clean amp ( version of the Steel String Singer). The amp is running 4 x 6550s.
I looked underneath and the amp uses 2 chokes for the B + for the screens. Each choke is a clone of fenders Twin reverb choke (90ma).
The only thing I am not sure about is if they wired both chokes in series to all 4 tubes or if they split the B + so that 2 tubes go to 1 choke and the other 2 tubes go to the other choke.

This amp sounded really really loud and stayed very clean!
Here is a back pic showing the 4 x JJ 6550 tubes.

Cheers
Guy

CustomClean.jpg

[M Fowler]

I just run the 6550's at high voltage no dropping. I always wanted to either buy a PT with plate and screen taps but didn't.

The original amp ran four KT88's but those tubes couldn't take the high screen voltage so I switched to 6550's and the amp has ran for a year that way.

I really should knock that screen voltage down to about 350v some how.

Mark

[erwin_ve]

Is it possible to add a parralel filter node in the ps right after B+ with 24k dropping resistor. That would knock down the voltage around 150v and the node is filtered as well.

[Bombacaototal]

New video

https://youtu.be/Fx_DuHrTt6g