Help me add some Dumble flavour to my old Hohner amp

[sumac]

Hello all,

first time poster here. I have this old german amp from 60's (Hohner orgaphon 30MH).  It is an all tube, four channel combo amp with reverb and tremolo sporting two fixed biased PL84's in push-pull configuration in output section, paraphase inverter and a lot of ECC808 preamp tubes :) here is the schematic http://i866.photobucket.com/albums/ab227/sumac2/hohner%20power%20amp/DSC04068_zps79241e60.jpg
When I got the amp, it was working but in need of serious overhaul. I replaced all e-lytes which helped with the hiss and hum, but the amp was still sounding kind of wrong. In the end I dissasembled the whole thing and plan to rebuild it. My plan is to keep the output stage and the guitar (first) channel (maybe change the tone stack for something more usable) and reverb and trem stock. The other three channels are however nothing special for guitar use and I want to replace them with a switchable "dirty" channel with a dumblish flavour and I have 5 free triodes for that. There is quite a few projects in the schematics section of the forum which use dumble-like preamps, so maybe I could just build one of those preamps and see how I like it. Have no idea which one should I go with though or how are they different. Another thing is how is this going to work with a paraphase and PL84's. I appreciate any comments and ideas..

thanks!

[thermion]

Cool...PL84. Had to look that one up. Not sure any grumble amps had paraphase inverters but it shouldn't be a problem to cobble the preamp you like onto this type of PI. Think about how you plan to accomplish switching, do you want a relay-based system built-in a'la mezza booger? It may be easier to just have discrete inputs and employ a a/b/c switcher in front of the amp, I dunno. Grumbles originally were just very well built bender-style amps with some preamp options and other tweaks.
You may not get much headroom with these output tubes, looks like PL84 is a 170 Va max from a spec sheet I found. Perhaps in your modding you could use a voltage doubler for the power section and run EL84, then use the un-doubled voltage for the preamp?

[sumac]

thanks for your answer!
switching.. I thought I would just use (four-pole) toggle switch - so that I can switch both input and output from the preamp at the same time. Should work fine? I would like to avoid mixing channels via mixing resistors.
PL84.. yes I've seen that some datasheet says 170V max on anodes, but reality is very different! :) B+ in my amp sits around 380V stock! the amp should put out 25W clean watts easily. BTW the output tubes in my amp are original - almost 50 years old and they seem to run fine!

[thermion]

Huh...over double Va max. I guess they don't make 'em like they used to!
As far as switching, it seems like you may need a lot of "back n forth" wiring to accomplish this with a 4-way switch? Preamp is low level signal and any noise intercepted by this wiring will get amplified by the whole rest of the amp.

should put out 25W clean watts easily

not likely, but we can hope.
let us know what you decide about preamps...

[sumac]

thanks, I will go with the D'Mars ODS preamp without the FX - I've got 5 triodes so that is ideal (will replace that mosfet follower with a tube one)
well the PL84's are no longer made, but they were intended for TV use (maybe that is why they are fine with higher voltage in audio circits?) so there is still a lot of them NOS around for cheap - nobody needs them anymore..
as for power.. I actually got original manual for my amp which says the amp put out 22Volts into 16Ohm with less than 1% distortion which is 30W!
will let you guys know how this preamp work with the paraphase..

[PRR]

PL84 is EL86 with different heater. Which sure appears to be a high-current EL84.

It's a V-scan TV sweep tube, also audio output (V-scan is a 50Hz ramp-wave, must have overtones past 5KHz--- "audio".)

170V is a "Typical Operation". That's what you get in a basic TV set (in europe).

250V is the Design Center max rating in TV service. Design Center can be exceeded 10%-20% if you know your voltages. And TV service is usually very derated from where we can run audio amps. A TV sweep runs max all the time, and a slight misadjustment can be a large increase of stress. Speech/music amps don't run flat-out for long.

380V and 25W is well above the maker's suggestions. However the makers sometimes didn't tout all the possibilities, steering bigger applications to higher price tubes. But when cornered, and teased with a big order for an under-sold tube, they'd work with a maker to find the outer limits of performance and profit.

Ah!! 400V 24W is the suggestion for 7189. Which IS an EL84 only rated 400V.

I suspect the "250V" spec was a CYA number in case they ever had to use the cheap plate-stuff. But when pressed, they admitted they never had to use the cheap stuff. Many EL84 survive 400V all day long (only if biased cool! not cathode-bias). If you needed a 400V number, you paid for 7189. Apparently Hohner felt safe using PL86 at the higher voltage.

[tubenit]

I think this is going to be a very challenging build.  I personally would attempt it with an LTPI instead of the concertina phase invertor.

Layout will be VERY important to avoid oscillations.

Here is the Tweed BluezMeister  and D'Mars  preamps using 12A_7's in a layout.  You'll have to figure out how to use a triode instead of the mosfet on the D'Mars layout if you go with that plan.

With respect, Tubenit