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Old 16th January 2012, 04:45 AM   #101
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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 by mr2racer; 16th January 2012 at 04:47 AM.
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Old 16th January 2012, 08:56 AM   #102
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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.
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Old 16th January 2012, 07:33 PM   #103
jgf is offline jgf  United States
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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.
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Old 17th January 2012, 03:50 AM   #104
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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
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Old 19th January 2012, 04:29 AM   #105
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Hey,
The heaters are good. I have 6.36 volts AC across the El 84s and 11.70 volts DC across the rest.
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Old 20th January 2012, 01:08 AM   #106
wa2ise is offline wa2ise  United States
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Quote:
Originally Posted by mr2racer View Post

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

Click the image to open in full size.
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).
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Old 21st January 2012, 03:22 AM   #107
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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
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Old 21st January 2012, 08:59 PM   #108
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The Harman-Kardon A-50K/A500 uses the output cathodes (7355's) as the source for the phono level tube heaters also.
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Old 22nd May 2012, 01:12 AM   #109
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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.
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Old 22nd May 2012, 01:54 AM   #110
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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
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