• 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.

El Cheapo, builders thread

Joku

Member
2009-05-17 9:02 pm
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?
 
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Joku

Member
2009-05-17 9:02 pm
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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Old Big

Member
2016-07-05 8:44 pm
Hello All,
Please forgive my ignorance and lack of knowledge but is the phase splitting done in the power tubes because I was going to build this amplifier with four 6ab4's. It's easier for me to wire that way and don't know if I have matching/ability to check triode on the 6ab4's?
 

Old Big

Member
2016-07-05 8:44 pm
Thanks for the replies guys!
I didn't make myself clear as always! I have read where phase splitting can occur in a power tube which I will be using 12aq5 for the power tubes.
I would like to use four 6ab4's (1/2 a 12at7) as the driver tube and was wondering if they need to have matching triodes? I guess with CCS they don't need to match because the current is regulated.
 

6A3sUMMER

Member
2016-06-07 6:50 am
Two cathodes that are connected to a common current source will have different currents, unless the tubes are very well matched.
Two cathodes that are connected to a common self bias resistor will have different currents, unless the tubes are very well matched.

Check what schematic you are using, to see if the cathodes are connected to either the same current source, or to the same self bias resistor.
 
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AmadeusMozart

Disabled Account
2009-10-08 10:29 pm
New Zealand
Two cathodes that are connected to a common current source will have different currents, unless the tubes are very well matched.
Two cathodes that are connected to a common self bias resistor will have different currents, unless the tubes are very well matched.

Check what schematic you are using, to see if the cathodes are connected to either the same current source, or to the same self bias resistor.

@6A3sUMMER

Cannot remember where I read it but if you have a double triode like the 12A?7 (with separate connected filaments) and the triodes are not extremely unbalanced then you can match them by slightly changing the filament voltage for each triode.

Please be aware that DC and constant current filament supply shorten tube life. See attached article, although it is for ordinary incandescent lamps it is just as applicable to vacuum tubes. (page 5)
 

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