• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

SCA-35 Dynaco

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hey All,
I think its easier than I thought to get those resistor values. Just use the values for the ST and start with the SCA 22k resistor for the phono section and then adjust that resistor via ohms law to get the right voltage on the 12AX7's.

Has anyone ever used a DC heater supply for the preamp tubes? would there be anything to gain by it?

Kevin
 
Last edited:
dcgillespie,

Thanks for your explanation. I would like to change coupling caps later (ie. Auricap) to see if I feel the difference. After that I hope to build a EL84 PP with different schematic (Now I have a pair of Hammond 1650 ouput transformers). Finally I want to compare both amps.
 
Haven't been on here in a few days...

mr2racer: your welcome. I used stock 50ohm value between the first
two caps. How are you liking the choke in place of the resistor? I'm
thinking of trying that. The advantage of DC heaters would be lower
noise, try with AC first (use 7025 or 12ax7 with spiral filament) and
see if its low enough.

An aside on hum: when I first installed the EFB mod (stereo 35 repro
boards) I had an increase in hum, or at least it was more noticeable.
Messed around with heater referencing schemes to no avail. One of the
heater pads for the 7247 is fairly far from the socket, I cut that
trace close to the socket, twisted the wires tight up to the center of
the socket, then soldered them in right at the pins. That fixed it.

tofsound: I put Solens in my amp initially (when it was still SCA35
circuit) and the cover melted back, caps were ok, but I chose to be
cautious and installed sprague 716p caps, which are rated to 105 deg.
C. It gets really hot in among the 4 power tubes. Most of the
boutique caps I've seen don't have a temperature rating published.
 
Hey All,
I think I've found a way to wire the heaters which will give me DC on the high mu 12xxx tubes. There isn't enough current from one of the two windings to supply the EL84's from one winding. So I intend to parallel the two 6.3 windings onto a terminal strip. Connected to that will be all four EL84's in parallel. At the same connection will also be the pilot lamp. And then at the same point two AC leads to a PAS style unregulated supply. I'd like to use a regulator but I'm afraid the regulator would cost so much voltage there wouldn't be enough across the heaters.

Does anyone know how much variation there is on the heater voltage for 12xxx series tubes? The PAS uses a voltage doubler. Assuming the same voltage drop as the PAS I'd end up with about 5.7 volts across the heaters? Is that enough? Would the circuit amperage rise under load to achieve similar power through the heaters?

Kevin
 
Has anyone ever used a DC heater supply for the preamp tubes? would there be anything to gain by it?

Kevin

I removed the cathode resistor for the output tubes, and used the phono tube heaters as a cathode resistor, to have DC for those heaters. See SCA 35 modifications

An externally hosted image should be here but it was not working when we last tested it.

Thinking of a way to get DC for the 12AX7s in the SCA35 preamp, to reduce heater hum. By passing cathode current from the output tubes. Yes, the old "preamp tube heaters as cathode resistors" trick. The four 6BQ5s look to be drawing 147ma of current thru the 95 ohm cathode resistor, to get about 13VDC. That current is close to that used by 12AX7s in 12V mode, but with 2 12AX7s I'd need 25.2VDC. But I could make that happen if I place a positive bias (in reference to ground, not the cathode!) on the grid 1's. And do that by grabbing the voltage between the 2 12AX7 heaters in series (see diagram). That will make the top end of the 2nd tube's heater be at 25.2VDC, or 12.6V above the grid bias. This is a little like the trick used to make a 5V regulator chip produce say 9V output (the tube cathode = regulator output, tube grid = regulator's "ground" pin, which is biased by a resistor voltage divider; which here are the preamp tubes' heaters).
 
wa2ise,

Neat trick. But I'm not sure that would work with Dave's EFB circuit. The PAS DC heater supply feeds four 12AX7's. With this amp I have three 12AX7's and one 12AU7 (2 12DW7's in the power amp boards) so the PAS circuit works for the phono board and the preamp tubes in the output section.

But wouldn't the voltage change as the cathode current changes with output?

Kevin
 
Hello,

I have done the EFB installation, Cap board replacement, bypassing the tone control with great success and now it's time to do something for phono stage. At the beginning I did not care about the phono stage but now I'm going to use it. I also want to replace the rotary switch and the volume pot with rather nice things because they are old and make some ploblems.

Here are my questions;

1. I would like to make the phono stage simply get one RCA input(for mm cartridge) and just connect the output to selector. I would like to cut out everything unnecessary but switching arrangement make me confusing.

2. Any mod to upgrade the phono stage ?

Thanks,
Kim.
 

Attachments

  • Dynaco_sca-35_phono_stage.jpg
    Dynaco_sca-35_phono_stage.jpg
    77.4 KB · Views: 317
There are certainly lots of variations of this classic design problem - how to best make an RIAA stage from a single 12AX7. My personal favorite is to run the first stage wide open, with either battery bias or an IR LED from the cathode to ground, and an anode follower second stage with RIAA in the feedback. It's surprising how high the second stage's feedback components' impedances can be without contributing significant noise. I like a first stage anode load of 330K, a series resistor of 452K and RIAA values of 100pF/750K and 330pF. (The second stage runs up to open loop below 50Hz.) Interstage coupling cap of about .022, large output cap.

All good fortune,
Chris
 
Yes tofsound that's what I did.

I have some questions if anyone can help? I wanted to use DC for the preamp filaments in the phono section and for the 12DW7 preamp tubes in the output section. (I'm using ST35 boards.) There wasn't enough amperage to simply use one winding for the preamp and one for the outputs. So I paralleled the two windings and then split them to run AC to the EL84's and built a rectifier for the preamp tubes. Also, I removed the 'hum adjust' pots. The question is should I use a ground after the last tubes? Ala PAS3 and ST35? Or put the pots back? I also intend to use Dave Gillespie's EFB biasing system.
 
The classic recommendation is to bias the heaters up to about +25 volts DC, or the factory original way of biasing up to the output tubes' cathodes. You'd only need one of the original hum pots if you wanted to try trimming. And why not? You've got the thing right there!

All good fortune,
Chris
 
The correct device to use in this application is an LM337 not LM377. (typo-?)
In the TO-220 package, from the front with the tab facing up, the leads are numbered 1/ ADJ, 2/ INPUT, 3/ OUTPUT. If you wired the regulator otherwise, and powered it up, I'd consider replacing it.
 
I thought I would chime in here. I restored and modified my SCA-35 using modules from Audio Regenesis. Some here might feel this is not "true" DIY, but I just wanted high quality components in my rebuild since I was going to put a lot of time into restoring this amp. This amp in stock form wasn;t in the best of shape and needed new boards anyway.

Dynaco SCA-35, 5.jpg Dynaco SCA-35, 7.jpg

I purchased the power supply/EFB module with an implementation of Dave Gillespie's EFB circuit. I've implemented the EFB mod on my Dynakit Stereo 35 power amp, and it was a great improvement, so I knew that I could get great results on implementing the same on an SCA-35. Moreover, the Audio Regenesis module is an elegant solution combining the EFB circuit with better power supply capacitors.

I also purchased the PC-10A and PC-11 amplifier modules. I modified my phonostage board according to Adrian Chan's post on Audio Asylum:

Re: Dynaco SCA-35 phono input stages - Adrian Chan - Tubes Asylum

This is basically an adaptation of Joe Curcio's modification to the PAS3 phonostage. Further improvements included shielded Beldon plenum cable and Mil-Spec Copper wire, a new set of tubes including a pair of Sovtek 12AX7LPS tubes from Jim McShane. I added a few of my preferred caps and resistors here and there to my liking. I put a ton of work into cleaning up the chassis, reconditioning all the pots and switches and rewiring the unit.

Dynaco SCA-35,13.jpg Dynaco SCA-35,15.jpg

For my analog front end, a friend donated a Kenwood PC-400U belt/idler wheel drive turntable. He bought off a seller from eBay who didn;t properly pack the turntable and the acrylic cover was shattered into multiple pieces. The turntable itself wasn't too bad, and needed some care to bring it back to life. I focused on restoring the drive system, and fabricated new thrust bearing plates for the main bearing and the belt/idler wheel pulley bearing. I also rejuvenated the rubber on the idler wheel with MG Chemicals Rubber Renue compound. I polished the bearing shafts and sleeves as best as I could and then applied Honda Molybdenum grease to the main bearing and Shell Formula 5W-20 oil to the belt/idler wheel bearing.

PC-400U, 3.jpg PC-400U, 4.jpg

I installed a Shure M97eX MM cartridge on the table and properly set up the balance, alignment, and tracking force. Together with the SCA-35, this is analog heaven.

2012-10-06 09.44.38.jpg
 
You did a beautiful job on the SCA-35. I'll bet it sounds even better than it looks! Congratulations! BTW what's that odd looking unit on the shelf under the turntable? Doesn't look much like a Dynaco item to me! :D

You are correct. That is my Audio Research LS-7 line stage preamp. It's in mint condition and I bought it for $575 from an ARC enthusiast in the San Francisco Bay Area. She wanted to make sure that she was selling it to a caring buyer (s0 I guess I qualified). I use that with my restored and modified Dynakit Stereo 35 power amp. Eventually, I will have two complete systems with analog sources. I am in the middle of restoring and modifying an AR XA with a Grace 707 MkII tonearm. I'll need to build a suitable phono stage to feed the ARC LS-7. For now, everything is in the man cave, but the second system will go in the living room.
 
Wow, that looks great, excellent work! I'm sure it sounds great too, the EFB mod really transforms it. One question: Should those PECs (packaged electronic circuit) be on the phono inputs after the phono stage mod? I read those instructions, but it's not clear to me...
I thought I would chime in here. I restored and modified my SCA-35 using modules from Audio Regenesis. Some here might feel this is not "true" DIY, but I just wanted high quality components in my rebuild since I was going to put a lot of time into restoring this amp. This amp in stock form wasn;t in the best of shape and needed new boards anyway.

View attachment 305843 View attachment 305844

I purchased the power supply/EFB module with an implementation of Dave Gillespie's EFB circuit. I've implemented the EFB mod on my Dynakit Stereo 35 power amp, and it was a great improvement, so I knew that I could get great results on implementing the same on an SCA-35. Moreover, the Audio Regenesis module is an elegant solution combining the EFB circuit with better power supply capacitors.

I also purchased the PC-10A and PC-11 amplifier modules. I modified my phonostage board according to Adrian Chan's post on Audio Asylum:

Re: Dynaco SCA-35 phono input stages - Adrian Chan - Tubes Asylum

This is basically an adaptation of Joe Curcio's modification to the PAS3 phonostage. Further improvements included shielded Beldon plenum cable and Mil-Spec Copper wire, a new set of tubes including a pair of Sovtek 12AX7LPS tubes from Jim McShane. I added a few of my preferred caps and resistors here and there to my liking. I put a ton of work into cleaning up the chassis, reconditioning all the pots and switches and rewiring the unit.

View attachment 305841 View attachment 305842

For my analog front end, a friend donated a Kenwood PC-400U belt/idler wheel drive turntable. He bought off a seller from eBay who didn;t properly pack the turntable and the acrylic cover was shattered into multiple pieces. The turntable itself wasn't too bad, and needed some care to bring it back to life. I focused on restoring the drive system, and fabricated new thrust bearing plates for the main bearing and the belt/idler wheel pulley bearing. I also rejuvenated the rubber on the idler wheel with MG Chemicals Rubber Renue compound. I polished the bearing shafts and sleeves as best as I could and then applied Honda Molybdenum grease to the main bearing and Shell Formula 5W-20 oil to the belt/idler wheel bearing.

View attachment 305845 View attachment 305860

I installed a Shure M97eX MM cartridge on the table and properly set up the balance, alignment, and tracking force. Together with the SCA-35, this is analog heaven.

View attachment 305846
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.