SCA35 ? & Ideas

[Hi-Fi Low-Life]

Hi Boys, I know hardly anything about electronics, but with solder gun in hand I’ve rebuilt the amp boards in my SCA35. I found this mod suggested on the Audio-Note site: “…if the original Dynaco 7199 driver is worn out and you replace it with a Sovtek 7199, the amp will oscillate at 500 Hz. This is because the feedback set-up in the SCA-35 is a bit quirky and only just stable with the original 7199, but not with the Sovtek which provides more gain. Detach the 1 uF C18 from pin 6, the pentode section's cathode, and connect it to ground instead. This will vastly lower the loop-gain and stabilize the amp once and for all. Plus, it will sound a lot better because of the much lower feedback. Plus, the Sovtek 7199 sounds better than the original Dynaco.”

Is this a good mod to do?

Also, I’m not planning on using the phono pre-amp board. It looks like the phono eq might be handled by passive components connected to the input jacks. If I take out these passives, could the phono board be converted to a zero gain tube buffer stage that isolates the volume control from the source components connected to the SCA-35? If this is possible, what would be involved? Alternatively, would it be possible to use the board as a headphone amp? These are probably nutty ideas, but what the hey... I thought I'd ask. Yellow_Light_Colorz_PDT_02

Thanks for any info you can share!

[Shannon Parks]

Depoe Bay, huh? I was just there last year. Lovely place!

I, too, have a SCA35 on my bench right now that I'm trying to get "up to snuff". Not having much luck yet. I just love the smell of my iron scorching that old plastic covered hookup wire. Yellow_Light_Colorz_PDT_42 So far, I've bypassed the tone controls, but still having noise problems. I'll keep an eye on these mods you mention. I'd like to keep my phono section, and I'm curious how it performs.

BTW, I tested about eight used 7199s from my collection. They are all over the place. Hopefully the Sovteks are decent, but this oscillation worries me. Hopefully all the ST70 guys will upgrade to an all triode driver. Yellow_Light_Colorz_PDT_02

[Hi-Fi Low-Life]

I re-tubed with Sovteks and my SCA35 is dead quiet (all filters & tone controls etc. have been disconnected). However, after turn-on there is a loud squeal that rises in pitch for about 10 seconds then pffftttt disappears & all is quiet. Do you think this is the 500hz oscillation Audio-Note was referring to? The squeal sounds like it's about 500hz, but as soon as (I assume) the voltages stabilize after turn on, it's completely gone. Is this potentially dangerous to the amp? The amp didn't do this at all with old stock Dynaco tubes, but those tubes are shot & noisy. What gives? Anyone?

[TerrySmith]

I recapped / rebuilt a SCA-35 a couple of years ago. It had an oscillation of about 2 - 3 hz, you could actually see it in the speaker. I swapped some boards from another unit and it was gone!

There are several different ways to go with this project, but if you want to keep the SCA itself, my idea would be to replace the boards with new ST35 boards that use a 7247 instead of a 7199. Then get a replacement preamp board and make a simple linestage preamp out of it, doing away with the phono part.

This is just my $.02, as I HATE the 7199 tube. YMMV.

[Shannon Parks]

Do you think this is the 500hz oscillation Audio-Note was referring to? The squeal sounds like it's about 500hz, but as soon as (I assume) the voltages stabilize after turn on, it's completely gone. Is this potentially dangerous to the amp? The amp didn't do this at all with old stock Dynaco tubes, but those tubes are shot & noisy. What gives? Anyone?

Hifi,

First, I have a question:

Did you change the main coupling caps to a different value - eg changed them from .1uF to .22uF? Or any other values of parts?

Yes, an oscillation can fry parts. Also, removing that 7199 cap is hurting your amp's performance.

[Hi-Fi Low-Life]

I used all the original values for the rebuild. After the rebuild, using the original Dynaco tubes, there was no squeal at all. It's only with the new Sovteks that I get the turn-on squeal, in both channels BTW. Nothing else changed in the circuit except the tubes.

I went ahead & tried the Audio-Note mod and…. Ta-Da! No more squeal. However, I thought the amp sounded better before the mod. I guess if I’m going to use the Sovtek 7199, I’m going to have to leave the amp with the Audio-Note mod installed. Yellow_Light_Colorz_PDT_13

Has anyone got a better solution? Thanks!

[Shannon Parks]

This circuit does a couple things. Mainly, it serves as a type of cathode bypass circuit. But it is also meant as a 32V supply for the screen. Maybe the engineers got just a little too crafty here.

If you could, give us all the DC voltages on all the 7199 pins of both channels, both before voltages with the RCA ones and then with the Sovtek. Also, do you have some various resistors and capacitors handy? We might change those 1uF caps to .22uF caps.

[Hi-Fi Low-Life]

I really appreciate your help. I've got the amp wired into my system & have to catch up on some music listening, but I'll check those voltages just as soon as I can. Thanks again!

[Hi-Fi Low-Life]

Call me crazy, but I've really been digging the sound of this amp since I disconnected the feedback circuit. At first, I thought without feedback that the amp lost a lot of detail. Now, after some extended listening, I think that it just sounds a whole lot more relaxed... and WIDE open. Bass is very clean too. BTW, I'm using the amp to drive a pair of Fostex FX120s in vented cabs.

I found these mods somewhere on the net a while back (can't remeber where) and thought I might try 'em out:

"Convert the output tubes to triode mode operation, by placing a 100 ohm 1/2 watt resistor between pins 3 and 4 of each output tube, and disconnecting the ultralinear taps of the output transformers from pin 4 of each tube socket. This cuts the output power in half, but gives a very smooth clean sound. "

AND:

"Strap the input section of the 7199s into triode mode by
removing C18 and R33, and connecting pin 3 to the pin 2 (=9) trace with a piece of wire. This lowers the gain (no problem w/ the tone bypassed) and sounds better too "

Can anyone tell me if these are electrically sound mods that won't blow up my amp? THANKS!

[mesherm]

Are you talking about triode strapping an EL84 or an EL34? Pins 3 and 4 are the plate and screen grid on an EL34. Pin 7 and 9 are the plate and screen grid on an EL84. To triode strap the EL84 connect the 100 ohm resistor to those pins and disconnect the UL lead from the tranny on pin 9.
Triode strapping is a matter of personal taste. Some like it, some don't. I usually only do it when the output trannies have no ultralinear taps.
When I triode strap, I use a UF4007 diode in series with the 100 ohm resistor with the banded end toward the screen grid. It will improve your power and sound.

[Hi-Fi Low-Life]

Thanks for clearing that up mesherm. Following your tips, I did wire the amp for triode mode. Sounds FABULOUS! Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_09

[Hi-Fi Low-Life]

BTW, I did try strapping the 7199 into triode mode & found it to be a rather large step backwards in SQ. Who woulda thunk it? Yellow_Light_Colorz_PDT_23

[erichayes]

Hi All,

HFLL, if you scroll down the Tube 101 TOC, you'll find "Phase Inverters", in which is a description of the phase splitter Dynaco used in all its amplifiers. The Cathodyne PS, when designed properly, is as credible as any other form of phase inverter. Problem is, most DIYers don't really understand what's going on with it, and have problems with oscillation and general degradation of sound quality.

First off, contrary to what triode-heads believe, the pentode is not an evil tube; it just needs to be applied to circuitry with a little more forethought than a triode. Capriciously converting a pentode to a triode--especially one that was never intended to operate as a triode--is inviting all sorts of sonic mayhem to the party.

What you're probably hearing is the sound of a push- amplifier, wherein one output tube is doing most or all of the work not very well, while the other tube is just sitting there cut off (or very close to it). A tube's natural inclination is not to be a cathode follower. We force them to be by clamping the plate directly to B+ without a plate resistor. In a Cathodyne PS, we're splitting the output of the tube--½ to the plate, ½ to the cathode. But we need a BIG signal going into the grid to get a decent output at the cathode. Without going out to the lab and hooking up the 570, I'm going to guess that a triode-connected 7199 will probably have the gain equivalent to a 12AT7 or a 12AY7, which is not gonna get a pair of 6BQ5s high enough to clear the trees when driven by (essentially) a 12AU7.

[Hi-Fi Low-Life]

erichayes, thanks for directing me to your post regarding phase inverters... Very informative! :eureka: