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ST-70 with Edcor Transformers
ST-70 with Edcor Transformers
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Old 12th February 2018, 12:01 AM   #1
H713 is offline H713  United States
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Default ST-70 with Edcor Transformers

Still in the planning phase, considering various options for my power amp project (thanks for the suggestions guys!).

I was thinking of building a ST-70 design using an Edcor transformer, mainly the CXPP30-6.6k. A470 transformers are a little more than I'd like to spend given that I'm relatively new to this. What are the downsides to using a 6.6K 30W Edcor as opposed to the A470? Any changes I'd have to make to the design?

Also, would using a 30W transformer be a problem for this? I'm okay with running at slightly lower power than the 35W the ST-70 originally made.
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Old 12th February 2018, 01:34 AM   #2
Eli Duttman is offline Eli Duttman  United States
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Any time a GNFB loop is present, magnetic headroom is essential. Edcor's CXPP60-4K is appropriate. A 6.6 Kohm primary impedance is correct for 7591s and 6L6s.

Don't bother with vacuum rectified B+. AnTek's low cost AS-3T325 will do nicely as the power trafo.
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Old 12th February 2018, 03:25 AM   #3
H713 is offline H713  United States
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Would I be better off doing this design using the 6.6k Edcor?
Mullard EL34 Push-Pull Tube Amp Schematic (Dynaco A420 Transformer)

I've also heard a lot about the El Cheap Grande amp on this forum, but have failed to find a (working) link to a schematic.
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Old 12th February 2018, 03:28 AM   #4
audiowize is offline audiowize  United States
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I have not had good results with the bandwidth available from Edcor's PP output transformers.

Would you be open to doing triode wires finals?
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Old 12th February 2018, 03:45 AM   #5
H713 is offline H713  United States
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Could you elaborate on what you didn't like about the Edcor transformers? Also, what do you mean by "triode Wire finals"? Sorry, still a bit new to this.
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Old 12th February 2018, 03:59 AM   #6
audiowize is offline audiowize  United States
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In a push-pull transformer for an amp with a lot of negative feedback (like the ST-70), it helps to have very high bandwidth. I found that the Edcor push-pull transformers fell apart pretty early up top, certainly using A470 clones will give you better results. (they cost more for a reason)

If you triode strap the EL34s, you can remove the negative feedback and not worry so much about high frequency instability.
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Old 12th February 2018, 04:01 AM   #7
H713 is offline H713  United States
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Which Edcor transformers were you using? I'd like to keep the output transformers for this project under 200.
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Old 12th February 2018, 04:03 AM   #8
audiowize is offline audiowize  United States
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The 100W 5K ones. I have some smaller 3K units waiting to be tested soon.
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Old 12th February 2018, 04:07 AM   #9
H713 is offline H713  United States
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Do you think using a smaller Edcor transformer (30-50W) would help with this issue?
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Old 12th February 2018, 07:51 AM   #10
Eli Duttman is offline Eli Duttman  United States
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Quote:
I have not had good results with the bandwidth available from Edcor's PP output transformers.
Interesting. Yet if the archives are scanned, info. from a member who successfully executed Williamson style circuitry using Edcor "iron" can be found.

Quote:
Also, what do you mean by "triode Wire finals"?
Triode wiring means connecting the screen grid of a multi-grid type to the plate by either wire or a small resistance. Triode wiring always lowers the internal impedance of the tube and very frequently improves linearity. That's the good. The bad is that power O/P is halved or worse.

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Mullard style circuitry, like that previously linked, is an excellent choice. However, EICO's HF89 is (IMO) a better starting point. You will use the Edcor CXPP60-4K and AnTek AS-3T325 "iron" previously linked.

Notice the dissimilar plate resistors in the 6SN7 long tailed pair (LTP) phase splitter. That's a crude technique for establishing balance between the 2 sides. The modern method is to replace the "tail" resistor (R15) with a constant current sink (CCS) and use 1 value for the 2 plate resistors. The low cost 10M45S suits this role well. You will use this part as R11 and R13. The inductance of that wirewound part will cause an increase in gain as frequency rises, which will compensate for the HF performance issue AW mentioned. Give with 1 hand and take away with the other. FWIW, the technique is called "peaking",

The triodes in the 6SN7 and 6CG7/6FQ7 are "identical". It's your choice between the 2 types.

A decent multi-meter is absolutely essential. Do you have access to an oscilloscope? No access can be worked around, but brute force techniques will be employed.

More "mouthfuls" to follow.
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