18W amp with EF86, ECC83 and tremolo channels

[signal.glow]

Hi,

i got nice FBT head with lots of space, it doesn't sound good and has way too much power, so i would like to rebuild it to 18W Marshall like amp. This schematic is starting point:
https://drtube.com/schematics/marshall/18wr.gif

FBT consists of one chasis for front panel controls and preamp circuit, another chasis with circuit for driver and OT tubes along iron is on the back. Looks like this:
https://www.mercatinomusicale.com/mm/a_ ... 64046.html
https://elektrotanya.com/fbt_500-r2_sch ... nload.html

I'm versed at building studio gear like preamps, compressors and similar with tubes or sometimes transistors, but not guitar amps.
I love using EF86 as input tube for mics, guitars, almost anywhere when its gain and/or sound is needed. I'm pretty sure 18W amp with a channel like this would sound wonderful, although i would like to also have ECC83 and tremolo channels like in original.

I checked how it is done in many revisions of 18W, Vox and others, they have ECC83 or EF86 and maybe tremolo channel. It seems like adding EF86 channel to the other two would require mixing resistors like suggested here, old Mullard's application circuit for preamp shows it too: http://www.valvewizard.co.uk/resistive.html

Can anyone please help me with it? FBT also has tube reverb which would be nice but i can focus on it later, the same goes for better tremolo circuit...

Some might be interested in my experiences with EF86 microphonics and noise. I used to use NOS Tesla EF806, when people started faking them and prices went up i switched to EH EF86 (not gold pins). From batch of 10 tubes which are not already sorted, about 7 are good even for mic inputs regarding noise (biggest problem with EF86 at my work), in guitar heads they aren't as suspectible to noise and don't become microphonic as JJ EF806 if i put foam between speaker and head. In rare ocassions when they are, most of them are also too noisy. So EH comes close to NOS Tesla, Siemens and similar EF806/EF86 in terms of microphonics, noise is almost always consistently higher but still acceptable. Sound of EH is the same and i like consistency of them. JJ "EF806" doesn't deserve special quality marking because they are inferior even to EH,ime. I got premature failures which is most of times microphonics, noise, not so often electrical parameters. Haven't tried O rings because i feel tube must be good and shouldn't need any treatment beside burn in to work well. Another difference between good western NOS and EH is longer warm up time before EH starts to sound good, for example Tesla only needs a minute or so but it is not a big problem.

Cheers,

Mischa

[JMPGuitars]

Hi, welcome to the site!

I would recommend starting with schematics and layouts from the downloads section of this website.

To build an 18 watt with an EF86 in the first channel is not that crazy, and has been done before. If you look at the EF86 Lite builds, and compare them to the Lite 2b builds, you will see the differences in component values needed. I drew a version of each that are available in the downloads section (the turret boards drawings I made are almost interchangeable). The "Lite" builds are essentially 18 watt amps built with only the 1st channel.

Apply those changes to your choice of appropriate 18 Watt tremolo or TMB circuit available in the downloads section, and you'll be on your way. I know some other places have done this already, but there's usually issues with their layouts.

If I were you, I would consider fitting a new chassis to the cabinet to make your life easier. Reverb can be had as well, I usually do this with the "one tube reverb" method. This is space dependent.

Thanks,
Josh

[geoff 1965]

Hello Mischa,
here's a schematic and a layout i had saved of 18 watters using ef86/12ax7 preamps you might find useful

18-Watts-Unleashed-Schematic.GIF

oucxgg.jpg

[signal.glow]

Thanks a lot. I've searched quite a bit around this forum, especialy download but couldn't really put something together. First schematic helped me a lot, will check other combinations as told.

Cases are connected together by a bundle of wire terminated into what looks like tube socket, having rounded plastic with hole on the top for cables. Wire's lenght is about 500mm without any shielding, pretty hard to do it this way for one off.
Front top chasis is made of thick steel which is almost impossible to drill for sockets. Inputs/controls are on the front, preamp and tremolo tubes behind are separated by 95mm (mid to mid socket) and pointing downward. Beneath, touching front of cab is reverb tank.
Chasis on the bottom behind tank is also made of thick steel. Both of them make sure tubes dissipate heat very well, i see no signs of it, some tubes have rounded, probably steel sheets acting as shields. EI transformer is very, very heavy, 70W OT and steel cases probably radiate all around...

Separating psu wires from shielded signal runs and connecting them to back chasis with some kind of connectors might be possible. 95mm between tubes seems a lot to preamp guy like me. At least front chasis would benefit from having tubes closer together which means cutting holes only in this one. FBT was probably noisy when new...
I have to think about mechanical part for some time, the problem is sloped control panel on the top which can't be placed down.
First thought is fixating front panel and put the rest on single Al chasis. That probably means quite long runs from controls which might have to be well shielded, so not easy.

[signal.glow]

Hi,

i found psu computer extension cables which could easily be aditionaly shielded and connected between preamp and power chasis. Have to see datasheets although there are some that take power for heaters and HT (voltage and current). Something like this:

https://www.ebay.com/itm/8-Pin-ATX-EPS- ... 0005.m1851

https://www.ebay.co.uk/itm/20cm-8-Pin-t ... Sw1JVaFW3X

Original in chasis has 7 pins without shielding, 8 pins would allow shorter separate/shieded runs, i'm thinking of adding proper shielded wire for PI signal some distance from power ones. Building and maintenance should be quite easy this way.
Does it look like a good plan, anyone used it? I know it is used in 8ch tube mixer with 2 masters and iirc, line amps for eq. This is at 300V/48W at least, it takes heaters one the top of all of this. Couldn't ask which model it was, although looks very much the same.
I know computer cables are meant for low voltages, so datasheets don't mention higher voltages. Like many resistors, they have higher max V because companies often don't test for more...
Thanks for any reply.

Cheers,

Mischa

[geoff 1965]

hello Mischa,
how is your progress going? check out the 2nd tubedepot schematic on the oscilloscope DMM post,it has ef86,ecc83 and tremelo 18watt!