• 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


diyAudio Moderator R.I.P.
2005-11-24 1:47 am
Hey, I thought it might be time to have a dedicated builders thread

Any builders around ?


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Change the 6AQ5 to 6P1P and the 12AT7 to a 6N2P and you are near a Chinese amp. They work fine by the way. Good cheap tubes.

6N1P is more prevalent as the input tube though. Either will work, along with 6N3P and 6N23P (more expensive). Tweaking the cathode and plate resistors is required as you switch from 1P to 2P to 3P to 23P.....

Nice idea to isolate the channels with diodes in the ps, but plain 1N4007s are fine at this point as they are always in conduction and see no rectification per-se.

An-tek AN-1T200 works fine for both filament (yea I know, heater) and B+ if you use the 6V tubes. It has two 6.3V windings,so they could be seriesed and used for 12V tubes.

Hammond and Edcor both make reasonable OPTs.

Lots of options to make sourcing easier.
We built one. Based on a Scout LK48 donor the power supply is different, and we used EL84 strapped in triode, biased into Class A, no glable feedback.. CCS on the bottom of the LTP is a Morgan Jones style cascode. A real stunner.


We've never taken it out of service long enuff to make it pretty.

Part of the magic of El Cheapo is how the distortion signature of the ECC81 cancels that of the output stage.



diyAudio Moderator R.I.P.
2005-11-24 1:47 am
It would be helpful to builders if voltages were indicated on the schematic.

Triad N-77U, says 115V, 100VA
I suppose that will be 2x 115V

Im also confused about choke T/E C354
Schematic says 1.5H/0.2A
Is that just smallest possible ?
Ehh, Triad C-24X have been suggested
And that is 15H/0.02A

Hmm, what to look fore as optimal ?
What kind of specs do we need ?
The currently recommended choke is the Triad C-24X (1 H./240 mA.). The doubler's pair of LARGE 'lytics do a good job of crushing the ripple fundamental. However, the highly triangular ripple waveform contains overtones of the fundamental extending well up into the RF region. That trash, along with any SS diode switching noise, is the primary target of the choke. If you want to improve filter action, insert a LC section made from a RF choke and a 1000 pF. mica or NPO ceramic cap. between the doubler stack and the "standard" style inductor.

As for voltages, the needs of the 12AT7 based small signal circuitry rule the roost. The 'T7 triode sounds good with 200 to 220 V. on the plate and an IB of 3 mA. That makes a B+ rail slightly over 350 V. "perfect". By itself, the N-77U does not get there. Use 1 of the 2X "12" V. windings of a Triad VPS24-1800 to boost the B+ rail voltage up and the 2nd "12" V. winding to energize the heaters of the 12AL5 and the "finals".

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.

I recommend that potential builders read the Audiophile Talk thread that documents the project's history. Yes it's very lengthy, but (IMO) worth your while.


diyAudio Moderator R.I.P.
2005-11-24 1:47 am
Hey, I found that Lundahl etc gets expencive, so I have looked at this EU supplier
Seems theres lots of affordable trafos etc

Frag Jan zuerst- Ask Jan first: Roehren und mehr

When using 6V6(GT?), I thought why not make all filament/heater 6V
And maybe an adjustable pure SS regulated supply for the CCS curcuit
And maybe also on ECC81
And I would build mono amps

I suppose thats possible ?
When using 6V6(GT?), I thought why not simplify some, and make all filament/heater 6V
That might result in using a pure SS regulated supply for the CSS curcuit

I suppose thats possible

I also am thinking of something similar. I have a bunch of 6AQ5 and 6AL5 tubes, and the ECC81 can be strapped for 6v operation. What I don't have is 12v heater windings on any of my spare transformers.
But I would like to hear it from Eli as to whether this is a good thing.
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diyAudio Moderator R.I.P.
2005-11-24 1:47 am
I would also like to ask if it would be ok to use regulated dc for heaters ?

rotaspec, if dc heater is ok, then Broskie has a supply kit that can convert to any heater voltage you want

hmmm, below is a quote from Broskie site :scratch2:

Many topologies, such as the Aikido and Broskie and White cathode followers or cascode or Aikido topology, use triodes standing atop one another, which makes referencing the heater power supply to the B+ power supply a small chore. With one tube atop another and a single heater power supply, the top and bottom heaters cannot share the same heater-to-cathode voltage relationships. With such totem circuits, the safest path is to reference the heater power supply to a voltage equal to one-fourth the B+ voltage; for example, 75V, when using a 300V power supply. Referencing the heaters to 1/4th of the B-plus voltage ensures that both top and bottom triodes see the same magnitude of heater-to-cathode voltage. The easiest way to establish this voltage relationship is to attach the heater winding's center-tap to the "Heater Ref." pad on the PCB. If the heater winding does not offer a center-tap, then place two 1k resistors in series across the winding and attach the junction between both resistors to the pad labeled "Heater Ref." on the PCB.

Speaking of heaters, where is the heater power supply? There isn't one—well at least not on the PCB. Many power transformers intended for use in tube audio gear are woefully weak in heater winding current. Here is an example, a 6.3Vac @ 1A heater winding cannot deliver anywhere near 1A of DC current at 6.3Vdc. When using such a transformer with a 1A load, it's best to give up on using DC and settle for AC on the heaters. In addition, there is an AC-heater fan club out there, making the PS-5 just perfect for its members.


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The 12AQ5 was chosen for its low cost. Remember, this is a tightwad's project. ANY member of the 6V6 clan will work using unchanged signal path parts values. Again, the hybrid rectified B- supply was chosen for cost containment reasons. However, the "soft" start protects against having the 'T7 grids go positive, with respect to the cathodes, which is an undesirable event.

The only place DC heater power makes any sense is for the 'T7s and that's overkill. The tail CCS does more than provide a high AC impedance. PSRR is pushed way up. This is a power amp, not a phono preamp. Good layout, careful grounding, and proper lead dress are all that's needed to obtain a low residual hum level.


diyAudio Moderator R.I.P.
2005-11-24 1:47 am
However, the "soft" start protects against having the 'T7 grids go positive,

Thanks Eli

well, there also is the 6AL5 with 6V heater, and even cheaper :D

btw, for those(like me) who may have been confused about T7, it refers to 12AT7

The exstreme low cost met by your design have been impressive
I wouldnt mind a small increase in cost, without going completely wild
With such low cost the shipping alone will be a major player
And we may have custom fee etc
The circuit gives OK results in combination with guitar amp O/P trafos. However, it really shines, when good O/P "iron" is employed. Use the best O/P trafos your finances permit.

FWIW, an early builder used black plate RCA 12AQ5s, Mullard made CV4024s, and Dynaclone Z565 O/P trafos. His results were excellent. No surprise in that result, as good tubes, decent "iron", and a rational circuit design were combined.

BTW, the 12 V. versions of many types are less costly than the 6 V. versions. ;) Off the shelf, AES charges $2.70 for a 12AQ5 and $5.90 for a 6005/6AQ5W.

Oh yeah, the European designation for the 6AQ5 is EL90. If you can get Mullard made stock, at a favorable price, use 'em.
BTW, the 12 V. versions of many types are less costly than the 6 V. versions. ;) Off the shelf, AES charges $2.70 for a 12AQ5 and $5.90 for a 6005/6AQ5W.

Oh yeah, the European designation for the 6AQ5 is EL90. If you can get Mullard made stock, at a favorable price, use 'em.

You can also use (with the appropriate sockets and heater voltages):


And possibly others, but there's some real nice 12 watters listed there that are really excellent tubes that fit comfortably into the "El-Cheapo" philosophy!
HH Scott 380 transformers

Hi. I received as a gift power and output transformers of Scott's 380 receiver.
Can this set be used for el cheapo build with 6V6 tubes? I want to use russian 6p6s in triode mode class A. Primary of the OPT is 6k. They are TRA-8-5-3.
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Pardon my ignorance, but can 12/6AQ5's take the B+ this thing puts out? I know it's within the ratings of 6v6's, but it's well beyond the listed max plate voltage of a 12AQ5. The reason I ask is I have one of these guys on the bench and one of my 12AQ5s is arcing. I had a booster transformer wired into the B+ supply, it's out right now and my B+ is at 320 when it stabilizes... then that tube starts arcing and I power it off. I know the original thread on this went down and maybe there was a bit about lowering B+ for use with 12AQ5s in there...