Lower voltage Dynaco ST35 for 6P14P // 7247 replacement - diyAudio
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Old 6th August 2007, 11:32 AM   #1
methar is offline methar  Thailand
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Default Lower voltage Dynaco ST35 for 6P14P // 7247 replacement


Two questions that need your advice.

I have an pair of original DYnaco ST35 PCB and having many 6P14P around. However, the ST35 use much higher volatage (+370) than the limits pf 6p14P (300V). Any one here know any modified schematic that run the ST35 clone at lower voltage?

Also, on the ST35 schematic, Can we just replace directly the 7249 with 12AX7 (or 12AU7)?

The ST35 schematic are here:

Thanks much in advance!

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Old 6th August 2007, 12:28 PM   #2
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Join Date: Nov 2006
Location: Munich, Bavaria
Hi !

7249 ?!
You mean certainly the 7247.

This t00b can be replaced by the 12DW7. It's currently manufactured by JJ. See here

Power tubes:
The dynaco is a typical american amp. Tubes are raped with 70V more than max. ratings......

Let's put it this way: Good 6BQ5/EL84 should bear that voltage without problems. Cheap junk could result in a nice fireworks.

I think the russian 6p14p which are really identical to western EL84/6BQ5 will work fine. But, to play it save you can reduce the value of C8A and increase the value of R12. You don't necessarily need to reduce the value of C8A, but this will reduce the power which is burned in the resistor.

Regards, Simon
Терпенье и труд все перетрут
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Old 6th August 2007, 12:41 PM   #3
methar is offline methar  Thailand
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Thanks Simon. Yes, I mean the 7247.

With the original schamtic, If I use 250-0-250Vac tranformer, could you advise what the value of R12 to drop the vaoltage to (B+) ~ 280Vdc?

Also, with doing so, will the rest of circuit value need adjustment?

Thanks in advance.
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Old 6th August 2007, 01:38 PM   #4
arhi45 is offline arhi45  Romania
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Location: Orleans France
hello, Methat

You could try this schematic. You can do it on the ST35 board with another ecc81/2 added. the finals are soviet 6P14. (6P18 works too). You will have about 10-12W with a very good sound. My mains transformer is a 200volt, and the rectifier a 5U4 (You have a 22V drop on each diode). Hope this help
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Old 6th August 2007, 02:09 PM   #5
arhi45 is offline arhi45  Romania
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the file
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Old 6th August 2007, 08:13 PM   #6
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The 7247 tube is essentially one half 12AX7 and one half 12AU7 in a single bottle. You can't easily replace the 7247 with either, unless you completely rewire the boards. It would have been a lot easier if the two boards on the ST35 were right next to each other, rather than on opposite ends of the amp, separated by the transformers.

I've got some late 70s / early 80s vintage Russian 6P14P tubes with the Reflektor logo on them. I run them in my ST35 clone amp with about 405 volts on the plates. I can't tell you that I'd recommend this setup, but it seems to work for me. I haven't seen any fireworks (yet). Feel free to drop me an email or a PM for more details.

I've found I dislike tinkering with the power supply to try to get the voltages where I want them. In my case, I used the "wrong" power transformer - that's why my B+ ended up at 405 instead of 370. I'm concerned that adding lots of resistance to the CRC filter (in an effort to bring down the B+) will hurt the bass response. It'll also generate a lot of heat that I don't need under the chassis.

If you start using a lower voltage power transformer, pay attention to the voltages at the front end of the amp. I think it was designed to run the 7247 off a 320 volt supply (with appropriate plate and cathode resistors). Assuming you make no other changes, the 250-0-250 unit you propose will certainly result in low front end voltages, which may shift the operating point of the AF amp and/or the PI into an undesirable region.

If you still have your heart set on messing with the power supply, download a copy of Duncan's PSU Designer II. You can simulate lots of different supply arrangements, and see where the voltages end up.
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Old 8th August 2007, 04:55 PM   #7
methar is offline methar  Thailand
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Thanks all for your advice. I will see if/what I can do.

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