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El Cheapo, builders thread
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Old 2nd November 2012, 01:38 AM   #21
Eli Duttman is offline Eli Duttman  United States
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The 'AQ5 can take the B+. Don't forget the cathode resistor drop. What kills tubes is exceeding dissipation limits, not Volts. The big difference between an 'AQ5 and a 6V6 is screen grid fragility. That's why the g2 resistors are 1 KOhm, not 100 Ω.

FWIW, I suspect you have a "touchy" specimen.
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Old 2nd November 2012, 09:11 PM   #22
patrickcurran21 is offline patrickcurran21
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Ah, thanks so much for the response
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Old 27th April 2015, 08:34 PM   #23
Joku is offline Joku  Finland
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Quote:
Originally Posted by Eli Duttman View Post
Builders residing in "240" VAC zones can use the Triad power "iron" mentioned, as both the N-77U and the VPS24-1800 have dual primaries. Unfortunately, the Allied 6K27VF is strictly "120" VAC. Builders outside of North America need to find something else to power the B- rail. The requirements of the B- rail are modest. About 30 V., to power the CCS, and sufficient extra Volts to allow the 'T7 cathodes to follow the negative swings of the I/P signal. 40 V. should be plenty. Please observe that the schematic drops 90 V. of B- in resistors. The 6K27VF was chosen for its cost control virtues.
Sorry about resurrecting this old thread, but I finally started my "El Cheapo" project, and ordered the trafos.

Anyways, the 10k resistor between the cathodes of ECC81 and the constant current sink seems quite high, as this will cause 60V voltage loss with current of 6mA. In addition, if we assume B+ voltage of 355V, the 50k anode resistors on ECC81 will cause further voltage loss of 150V.

If I have B- voltage of say, 35 volts, would a smaller resistor, say 1k, be a better choice?

Last edited by Joku; 27th April 2015 at 08:37 PM.
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Old 27th April 2015, 08:56 PM   #24
Eli Duttman is offline Eli Duttman  United States
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Joku,

The resistor between the 'T7 cathodes and the CCS performs 2 functions: dropping B- volts, and thermally isolating the SS parts in the CCS from tube heat. With B- down at about 35 V., use a 100 ohm carbon film part, as you have no volts to spare. All your build needs is the thermal isolation.
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Old 10th May 2015, 09:57 PM   #25
Joku is offline Joku  Finland
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I started assembling it, but I started to get some weird oscillation when the global negative feedback wire is attached to output. However, when I removed the wire closing the negative feedback loop, it seemed to work fine:

Any idea where I should start debugging? I'm using Hammond 1650F output transformers.
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Last edited by Joku; 10th May 2015 at 10:10 PM.
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Old 10th May 2015, 11:16 PM   #26
Eli Duttman is offline Eli Duttman  United States
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You may have phasing wrong and are applying positive feedback. Try criss/crossing the O/P tube plate connections.
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Old 16th May 2015, 07:10 PM   #27
Joku is offline Joku  Finland
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Yes, that was it. Now the amp is constructed and working well. However, is it normal to have some blue glow in the tubes:
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Old 2nd December 2018, 01:22 PM   #28
analogadikt is offline analogadikt  India
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Have gone through this thread and have a question. The Z565 has been suggested as a good OPT to use for this. That is an U/L trafo but the schematic does not show any such connections. Is it to be used without any connections to those U/L taps?

Regards,

Anwesh
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Old 2nd December 2018, 03:15 PM   #29
Eli Duttman is offline Eli Duttman  United States
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If you want the extra power UL mode yields, connect the 1 Kohm screen resistors to the UL taps, instead of the plates.

It is possible to add mode switches. If you do, change mode ONLY in the unpowered state.
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Old 2nd December 2018, 05:16 PM   #30
analogadikt is offline analogadikt  India
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Thanks Eli

Anwesh
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