Harmonic Tremolo Amp Build

[jrgtwo]

Hey all,

First post, just built a 6v6 push pull amp with harmonic tremolo and wanted to get some feedback on the design as well as any ideas on how to better fine tune the amp.

Its a mish mash of schematics I was able to piece together, but its mostly a standard preamp -> DC coupled cathode follower -> hiwatt tone stack -> revibe harmonic trem -> reverb -> deluxe reverb phase inverter, psu, and output transformer.

It sounds about as good as I could imagine, but the reverb needs some work because its there but also seems a bit quiet even on full bore, and I think the tremolo pre amps could use 100k resistors instead of 220k.  It seems to crunch a lot more than I'm wanting

Let me know if you can't read anything in the schematic, this is only the 2nd or 3rd schematic I've put together, and maybe the 3rd point to point amp I've put together. 

V1 - 12ax7 - pre amp 1/preamp 2
v2 - 12ax7 - cathode follower/ trem high
v3 - trem low / lfo inverter
v4 - 12ax7 - lfo oscillator/ second half not used
v5 - 12dw7(ecc832) - reverb
v6 - 12at7 - phase inverter
v7/v8 - 6v6gt

Sound Cloud Tremolo examples
https://soundcloud.com/jonathan-garcia-660694352/sets/harmonic-trem-amp











Learned lots from here https://www.tdpri.com/threads/exploring-fenders-harmonic-tremolo.791406/

-Jonathan

[HotBluePlates]

... the reverb needs some work because its there but also seems a bit quiet even on full bore ...

I won't try to second-guess your 12DW7 (?) Reverb circuit.

But check out the 220kΩ mixing resistors right before the words "to phase inverter."  Those appear to mix your Dry and Reverb signals, right?  220kΩ & 220kΩ is 50/50 balance of Dry to Reverb.

Look at the Deluxe Reverb.  While reality is a little more involved, Fender's plan for mixing was 3.3MΩ from the Dry path (to a grid of V4), and 470kΩ from the Reverb path (found at the output of the Reverb pot, and right beside the 3.3MΩ on the layout).

These are voltage divider circuits:
     Dry: 470kΩ / (3300kΩ + 470kΩ) = ~12.5%
     Reverb:  3300kΩ / (3300kΩ + 470kΩ) = ~87.5%

So before anything else, you've "turned down" your reverb compared to Fender.

If you've also "wired the Reverb pot backwards" as-drawn (input on the wiper, output from the non-grounded end of the track), that will further reduce the Reverb signal and increase the Dry signal.

So, make sure the Reverb pot is wired right-way around with the output on the wiper, and make your resistor from the Dry path at least 1-3MΩ (after which you can consider whether the Reverb path needs a 470kΩ in place of the 220kΩ).

[jrgtwo]

Thank you for the reply, I will try updating those resistors, I should have some larger resistors to check with.  I'll double check the wiring of the pot as well.  Still new to drawing up the schematics, may have just drawn it up wrong. Usually just piece things together in my head and hope the amp turns on :)

The reverb circuit I used mostly came from this post https://el34world.com/Forum/index.php?topic=21991.0
and this article on guitar.com https://guitar.com/guides/diy-workshop/build-tube-spring-reverb-unit-amplifier/

[jrgtwo]

Thank you HotBluePlates

I replaced the dry signal 220k resistor with 1meg and the amp is now swimming in reverb.

I also replaced the tremolo preamp plate resistors with 100k which really cleaned up the amp.

Will post some new recordings when I get a chance

-Jonathan

[jrgtwo]

Here are some updated sound clips after the reverb mixing resistor update in case anyone wants to see what a difference it made.

Previous recordings had the reverb maxed out, these new ones have it at about 1/3 up

https://soundcloud.com/jonathan-garcia-660694352/sets/harmonic-trem-with-updated-reverb

The amp is driving a little too easy so still some work to do with this project, but happy with the progress so far

Thanks again HotBluePlates!

[HotBluePlates]

... Previous recordings had the reverb maxed out, these new ones have it at about 1/3 up

... happy with the progress so far ...

That's sounding pretty great!

Hopefully now you can envision those channel-mix resistors like "double-ended Volume pots."  They're not always the correct thing to tinker, but a good place to start when the levels of the things being mixed are naturally un-equal.