Dynaco SCA-35 Restore
- Thread starter Dave451
- Start date
Thanks for the recommendation, but I'm going to give the new productions a go for this round. I have had very good success with Sovtek EL84M's in my 299B's and the sound is excellent. That's why I am seeking recommendations for new production EL84/6BQ5, to see if I can find similar good sound with the somewhat broader offering of new production tubes that aren't necessarily 7189-rated, like I needed for the Scott amps. I have heard that the SCA-35 is hard on output tubes, but with Dave G's EFB circuitry in play and a CL-80 to knock down the line voltage a bit that should help.
Frankly, I'm also looking for something a little more economical than what I expect a quad of NOS would cost. I do have Sylvania black plate and Mullard on hand, but nothing that resembles a matched quad.
Dave
Frankly, I'm also looking for something a little more economical than what I expect a quad of NOS would cost. I do have Sylvania black plate and Mullard on hand, but nothing that resembles a matched quad.
Dave
Thanks, Peter. I am going to try the JJ's sooner or later. For now, I corresponded with Jim McShane on it. Jim prefers Sovtek EL84M's for unmodified SCA-35's because they are so hard on output tubes. With EFB installed, as this one has, he is more comfortable with straight EL84/6BQ5 outputs. I ordered and now have on hand a set of the Tung Sol Re-issue EL84/6BQ5's to try. Will give a report once they are installed and measured and listened to. Leaving for a couple weeks, so full testing will have to wait a bit!
Dave
Dave
Quick update before departure: I put the Russian Tung Sol's (EL84) in the SCA-35 and they biased very well at considerably less turning of the bias level pot than the worn NOS tubes. Dave G says to average at 270 mV per side and these tuned up at 267 and 273 mA--nicely matched quad from Jim McShane. I haven't run a power check, but clearly getting more out with the new tubes and sounding quite good--need to play them in a while, but really liking the sound into my Klipsch KG4's and a newly acquired set of Mission 770 Freedoms.
Dave
Dave
Glad to hear you are pleased so far, I just finished installing the preamp board in mine and will start the process of taking out the old driver boards next. There are some wires I need to replace also, previous owner spliced some going to the EFB board that was already installed when I received the amp.
Quick update: back in town now and been listening closely to the SCA-35 and I have to say I'm very impressed with this amplifier. The Russian Tung Sol 6BQ5/EL84 re-issues are opening up nicely, probably with a lot of other new components in the amp and the sound is quite pleasing: robust with great response throughout the range. I've been driving my Klipsch KG-4's with my Onkyo CD player and (excellent) Scott 350B tuner driving it to very good effect. Since it needs a little more input, I took the 6 dB attenuators out of the line for CD's and the 350B has plenty of fizz to drive it. The Klipsch have good sensitivity, but a surprise for me is the ease with which the Dynaco is driving my KLH Model 5's, which are quite a bit less efficient. Great low end (with a little help from the bass control) and the mids and highs from the Model 5's are just outstanding. The little Dynaco is more than holding its' own against my Scott 299B so far.
I'm going to power test the amp soon, but there's a ham radio on the bench that's resisting my charms right now (persistence is 3/4 of the battle trouble shooting mis-behaving ham radio transceivers). In the meantime, though, I get to just listen to the SCA-35. It's a tough job, but someone has to do it!
More when I have the power test figures.
Dave
I'm going to power test the amp soon, but there's a ham radio on the bench that's resisting my charms right now (persistence is 3/4 of the battle trouble shooting mis-behaving ham radio transceivers). In the meantime, though, I get to just listen to the SCA-35. It's a tough job, but someone has to do it!
More when I have the power test figures.
Dave
I wish I was having the same results, mine really sounds like crap at the moment. Installed new tubes and they biased nicely, plugged in a cd player which improved the gain some to where 1/2 volume is a decent listening level. But the sound is distorted at a certain frequency, probably mid bass area. Not sure where the problem lies, but I'm going to walk away from it for a time before going through all the components to make sure everything is correct. The only positives is there was no smoke or hum, dead quiet at idle.Quick update: back in town now and been listening closely to the SCA-35 and I have to say I'm very impressed with this amplifier. The Russian Tung Sol 6BQ5/EL84 re-issues are opening up nicely, probably with a lot of other new components in the amp and the sound is quite pleasing: robust with great response throughout the range. I've been driving my Klipsch KG-4's with my Onkyo CD player and (excellent) Scott 350B tuner driving it to very good effect. Since it needs a little more input, I took the 6 dB attenuators out of the line for CD's and the 350B has plenty of fizz to drive it. The Klipsch have good sensitivity, but a surprise for me is the ease with which the Dynaco is driving my KLH Model 5's, which are quite a bit less efficient. Great low end (with a little help from the bass control) and the mids and highs from the Model 5's are just outstanding. The little Dynaco is more than holding its' own against my Scott 299B so far.
I'm going to power test the amp soon, but there's a ham radio on the bench that's resisting my charms right now (persistence is 3/4 of the battle trouble shooting mis-behaving ham radio transceivers). In the meantime, though, I get to just listen to the SCA-35. It's a tough job, but someone has to do it!
More when I have the power test figures.
Dave
Sorry to hear it, Steve. I triple checked everything and despite my care I found a couple of errors in my wiring. Since I took the whole thing apart and put it back together different, it was tough in spots keeping track of what was original wiring and what changes were proper with the new circuit boards. Probably best to just let it rest a bit and come back fresh, as you say. It'll be worth it when you get it going properly!
Dave
Dave
I don't believe it has anything to do with the wiring since both channels are equal and producing sound. I think it might be a resistor or cap value in the wrong place or something. The only tubes that are not brand new are the 12AX7's, but they tested good, so I doubt it is tube related.
OK, power tested the amp with the new tubes. Re-biased first. The 270 mV had decreased to about 260 mV as the tubes played in, so had to adjust slightly to get 270 mV--very close on both pairs. The quad is well balanced. With single channel driven, got 17.4 WRMS on both sides. With both channels driven, power was 14.9 WRMS per side. Looks like full power with the new Tung Sol Russian re-issue 6BQ5/EL84's.
Also took a quick look at stability by testing each side at 10 kHz square wave into 16 ohm non-inductive load with volume control all the way up and input adjusted for 1w RMS (4.0 VRMS) output. Tone controls centered and balance shifted fully to the side being tested. The traces look pretty good to me, but would appreciate other opinions on these output waveforms (see shots below; left is first, right is second photo).
Dave
Also took a quick look at stability by testing each side at 10 kHz square wave into 16 ohm non-inductive load with volume control all the way up and input adjusted for 1w RMS (4.0 VRMS) output. Tone controls centered and balance shifted fully to the side being tested. The traces look pretty good to me, but would appreciate other opinions on these output waveforms (see shots below; left is first, right is second photo).
Dave
Attachments
Nice job, Dave! The square wave presentations are actually quite good for the fundamental frequency you used. Normally, a 10 kHz fundamental would be used on the power amplifier section only, whereas you are passing this signal through the tone control section first, which will not have adequate response for faithful reproduction of a 10 kHz fundamental. In view of that then, the presentation is really very good.
If you want to more closely check the accuracy of the tone controls, then frequencies of 200 Hz and 2 kHz will be more appropriate, with both frequencies then allowing a quick check for response over the full 20 Hz to 20 kHz spectrum. If you want to check just the power amplifier circuits, then set your controls as before, but inject a 10 kHz square wave at the wiper terminals of the balance control, and use the attenuator on your generator to control the amplitude of the waveform. The sensitivity will be very high at this point.
As for your power output measurements, they are slightly lower than I routinely measure, but there are multiple possibilities that could account for that, from test equipment calibration to AC line voltage the test was made under. With the 121 vac that is routinely available at my location and the use of EL84M output tubes, most examples will develop right at 17.5 watts with both channels driven at the onset of clipping. Ultimately, the discrepancy is small.
Congrats on a great rebuild of your SCA!
Dave
If you want to more closely check the accuracy of the tone controls, then frequencies of 200 Hz and 2 kHz will be more appropriate, with both frequencies then allowing a quick check for response over the full 20 Hz to 20 kHz spectrum. If you want to check just the power amplifier circuits, then set your controls as before, but inject a 10 kHz square wave at the wiper terminals of the balance control, and use the attenuator on your generator to control the amplitude of the waveform. The sensitivity will be very high at this point.
As for your power output measurements, they are slightly lower than I routinely measure, but there are multiple possibilities that could account for that, from test equipment calibration to AC line voltage the test was made under. With the 121 vac that is routinely available at my location and the use of EL84M output tubes, most examples will develop right at 17.5 watts with both channels driven at the onset of clipping. Ultimately, the discrepancy is small.
Congrats on a great rebuild of your SCA!
Dave
Thanks much for your response, Dave. Yeah, I was hoping to do a little better with both channels driven, but I'll take it. I'm wondering if the output power is the difference between the EL84M's and the Tung Sol 6BQ5/EL84 Russian re-issues I'm trying? In the future I may swap and see.
Next time I'm tuning it up, I'll do the post-tone control injection (thanks for the reminder) and see how it stacks up, but I was pretty happy with the 10 kHz response. Also the tone control 200 and 2000 hZ checks as well.
Thanks, guys, for the other comments as well. I too actually like the look of the satin black compared to the gray/brown of the original. Derekva, interesting note on the Telam / Polam tubes--I'll check these. LOL on the "Lazarus SCA-35"--now it has a name!
Dave
Next time I'm tuning it up, I'll do the post-tone control injection (thanks for the reminder) and see how it stacks up, but I was pretty happy with the 10 kHz response. Also the tone control 200 and 2000 hZ checks as well.
Thanks, guys, for the other comments as well. I too actually like the look of the satin black compared to the gray/brown of the original. Derekva, interesting note on the Telam / Polam tubes--I'll check these. LOL on the "Lazarus SCA-35"--now it has a name!
Dave
Yep--sounds like a good candidate for a rebuild. Hope yours is in better shape than mine starting out! Do heed the good advice I got above in keeping the wire dress as much like the original as possible to keep the hum down. It can be a challenge with AC heaters. Good luck with it!
