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scott17 3rd September 2012 08:50 PM

5CZ5/5BQ7A single ended amp
3 Attachment(s)
I wanted to make a stereo SE amp as small as possible. I had a few 5CZ5 and 5BQ7A that I decided to use. I used a dropping resistor for the heaters, but it would be easier to use the 6CZ5 and 6BQ7A. The 6CZ5 are $7, the 5CZ5s are $3 at The 6BQ7A are the same price, $3, as the 5BQ7A.

I don't think I could construct it to be much more compact than it is. The amp sounds very good. Less than 1mV of hum on the speaker terminals. I would guess it's about 5W per channel. I am very surprised with the response of Edcor XSE10-8-5K output xfmrs. Never used them before. Not too bad. The Antek toroid looks pretty good with the steel cover also.

I used a CL80 thermistor for the inrush current limiter because that's what I had. I think the CL120 may be more appropriate for this design.

This amp can be built for about $140, buying all the parts new, not including the chassis. I used some funky trim material I found at HDepot and a piece of scrap aluminum.

soulmerchant 3rd September 2012 09:49 PM

Nice little amp! you can by-pass the UF4007's with inexpensive low value film capacitors (use one for each diode) to help smooth the edges off the ripple. Better yet source a choke to get get the last of that hum.

scott17 3rd September 2012 11:22 PM

Thanks. Ha! I don't think I could fit a choke in that box!

My loudspeakers are 97dB and I can't hear a thing. The amp is dead silent.

I am interested in the bypass caps for the bridge though. What would you suggest? 0.01uF?

silcostoni 4th September 2012 01:14 AM

Look great ......i hope ,,i can get This Tube in Indonesia :)

scott17 4th September 2012 02:51 AM

FYI. The red LEDs that I used are just standard devices I had around and matched to achieve a 3.5V drop.

wa2ise 7th September 2012 02:25 AM

With the 5V heater tubes, you could use an old PC computer power supply to heat them. Of course you still need to build the B+ supply. :)

scott17 7th September 2012 02:38 AM


Originally Posted by wa2ise (
With the 5V heater tubes, you could use an old PC computer power supply to heat them. Of course you still need to build the B+ supply. :)

That is true, however look how easy it is the way I've done it to keep it compact.

Besides that the heater voltage for 5CZ5 is 4.7V. The heater voltage for 5BQ7A is 5.6V.

The 5CZ5/5BQ7A combination sounds fantastic. I say it rivals an EL84/anything driver.

Inexpensive too.

scott17 20th October 2012 02:24 AM

The more I listen to it, this is a fantastic sounding amp, and very inexpensive. If anyone would like the Mouser parts list please PM.

bondini 4th August 2016 07:04 AM

5CZ5 or 6CZ5 SEUL - pleasant sounding
My build uses the 6CZ5 single-ended in ultralinear mode biased at -15V with a 5kOhm load and 285V on the anode. The 6CZ5 is driven by a 6AH9 configured as a Kimmel-Mu stage.

Distortion rises linearly with output up to about 3 Watts (THD is 5% at 1.5 Watts, 8% at 2.5 Watts) comprising mainly 2nd and 3rd order harmonics. It clips above 3 Watts after which odd-order harmonics dominate. All the distortion is in the output stage (the 6AH9 Kimmel-Mu is blameless). Output noise is 1.4 mV.

Initially, I used LEDs to bias the output stage - the sound at clipping was unpleasant as the LEDs flashed away. The bias is now a conventional bypassed resistor - clipping (that regular feature of playback) is now much less problematic. YMMV.

It should sound awful and it does sound awful when trying to drive speakers rated at 1 Watt 87dB. Pathetic.

It doesn't sound awful when driving 97dB speakers - in fact, it sounds very good. Listenable and enjoyable.

kylej1050 5th August 2016 03:49 PM

With step-up and step-down switcher stamps on eBay going for under $2 each that you can set to any voltage between 3 and 40V at a couple amps you can use almost any tube with ease.

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