ScottyDogg Haha, let's see how they turn out first. ?

I've made some progress on getting a consolidated circuit diagram together, rev 0.1 will probably be ready to post over the weekend. After that I'll be looking at layout to see if we can squeeze everything we're planning into a box!

We've had a bit of a change of thinking around this project and may look at making it a head instead of a combo... and since it's a head we may look at bumping it up to the full 50W with two EL34s in the power section. We're still looking at the power scaling (although we obviously loose the option of the half power mode) and I'm looking at adding a post-phase inverter master, in addition to the 'master' on the gain channel... which actually sits before the tone stack, so I'm thinking of it more as a volume.

Yeti I'm liking the sound of EL34's and a head form factor even more!

Yup. This thing is gonna be awesome with el34s ?

I reckon its a sound decision going with a head..

Right, so I've put together a first pass of the circuit (still needs a proper review so there are likely to be mistakes). The Pre-Amp is more or less a direct clone of the Quick Rod circuit, although I am tempted to tweak that clean channel a little more sparkly (possibly change the tone stack to something a little more Fendery).

The Aux page has the circuitry for the footswitch controller (using optocouplers... more reading required) and the FET FX loop. Both of these will be built on veroboard, probably with the jacks mounted straight on the board, which will fix the board against the back panel of the chassis (now that I look at this, the FET option, may not save that much space over an extra valve... but it will be cheaper). My current design has duplicate buttons on the amp and the footswitch so that channel, gear, etc. can be controlled from either... I'm not 100% sure if this is going to stay however because those switches add quite a lot of real estate and I'm worried about the wires running backwards and forwards across the amp, although this would be minimised if they were mounted on the back, near the footswitch plugs...

The Output stage is a combination of the Quick Rod inverter and a relatively generic cathode biased EL34 circuit (although, actual value of bias resistors may change). It also has a Post Phase Inverter Master Volume thrown in for good measure.

Power supply is relatively generic except for the added Power Scaling, I've included the chip number that I used for my 15W black face clone, if I remember correctly it should be able to handle the current but I will need to double check, current plan is to mount this directly onto the chassis for heat conducting. I'm aiming for the voltages specified on the QR schematic and may need to teak a few resistor values to get them but in my (limited) experience these aren't that critical for the pre-amp tubes, provided your power amps have the correct voltage. I've also included some pretty beefy capacitors and an over-sized power transformer. The reason for this is that I want a properly stiff power supply for this amp to prevent it from loading down. Incidentally, I've gone with a half wave rectifier simply because of the transformer I've found, if we get one wound, I'll probably look at halving the voltage and going bridge rectifier.

Next it's on to the layout, which will help us size the chassis and ultimately the head itself... this is quite a complex beast, so I expect it to be large.

p.s. on a side note, everything I know about grounding I learnt from the Valve Wizard. (http://www.valvewizard.co.uk/Grounding.html) For this amp, I'm planning on using the star grounding layout with chassis connections only at the AC plug and the input jack. Heater wires will be earthed via center tap on the transformer.

Hmmm... we also need to come up with a name for this project.

Nice. Thats looking great..

Nice. That is busy. Will be curious to see how it fits. At least we have some options if we need to reduce hardware

First pass layout is done. There are some things that I'm not 100% happy with (e.g. the proximity of the FX loop board to the tube beneath it) but I think that it's a reasonable proof of concept. Dimensions for this chassis are 525x200x60.
Current plan is to make the build of this true point to point and use as many terminals on the tube sockets and controls as possible. Intending to use turret board for intermediate points.

I think the next step will be to lay out the two veroboards because at this point their sizes are pure thumb suck.

Cool project! How is it all going?

So... on behalf of Yeti who is the brains behind this all.

The initial design is largely done. We have souced quotes for transformers which we will likely move to ordering in the next week. Then a revisit to check the components and layout so we can finalise the chassis. I have reached out to a few folk regarding getting the chassis made up.. looking at aluminium or mild steel sheets. We are aiming to bed down the layout so that hopefully we can bed down where the holes go so that the cnc can do it.

Once the chassis is sorted.. i will start moving on the head cabinet to house these. Reached out to some contacts for tolex.

Sooo... i think it is going along nicely.. if Yeti has the patience.. im am hoping to wire and solder mine alongside him to learn something in the process. But we will see how it goes ?

Looking good. I understand Aluminium would be better for heat dissipation.

Weeeeeellll, this is grinding slowly back to life.
We've just received the transformers we ordered back in Feb... 15kg between the two amps, that's a lot of iron and copper. These were custom made for us by Peter Souris Transformersin the Johannesburg CBD.

I've also done a preliminary layout of the circuit boards, going to take another pass or two before I post but we're getting there and I've made some good progress on specing the components and should be ready to place the order quite soon.
Also, with the transformers in hand, we can finalise the layout and start ordering the metalwork.

Nice. Could you share the transformer / choke specs and ballpark cost?

You wanted opinions on standby switches, I am ignorant about amps, but feel it is beneficial. I have two amps without, one is the 5F1 -alike with a tube rectifier. The current ramp-up on it is, well, not benign. The Laney Cub 10 has silicon rectification, not sure how they control the current inrush. Just seems better to switch to standby when unplugging stuff, or waiting for the President to finish the latest lockdown speech.

modulator
Specs and prices (ex Vat) as follows:

Power transformer - R840
Primary - 240V
Secondary 1 - 345V 400mA
Secondary 2 - 6.3V (with center tap) 5A

Choke - R180
5H 400V 120mA

Output transformer (suitable for push-pull 6L6) - R890
Primary Impedance - 3200 Ohm
Secondary Impedances - 4, 8 and 16 Ohm
Power - 50W

These were largely based on these Hammond transformers:
Power - 290GX ~ R2,008
Choke - 194F ~ R441.59
Output - 1750N ~ R1503

So I hear you on the unplugging... thing is, the standby switch pulls the high voltage off tubes' plates... which is a pretty drastic method of muting. If that's the goal (and given that we're talking scratch designed amps) then I'd rather add a mute switch that shorts out the inputs.

Thanks. I have no idea whether the potential and current inrush to the valves at startup is bad or not, I know my father's old Philips gram had no standby, and I remember him always switching on and waiting a few minutes before turning up the volume. I have that Philips here, as a youth I removed the valves and later the speaker (one really should become a Yedi and see into the future) and I really have to get myself in gear and fix it, or get someone to fix it. Uh, maybe first check if it does FM... Nevermind, with a vinyl-friendly turntable it should be fun, even if it is only a milliWatt and mono.

On the other hand, my ex-wife has a Stereo set here, and she says as kids they never worried about valve do-do-not's (did not know) and always switched on and rocked. Still goes, original components. I would love to fit a decent turntable in that one. So, no, I cannot really advise on standby switches.

Have fun.

According to my reading, the standby switch was added by one of the big guys (Fender?) because they had cut costs on the reservoir capacitors and they couldn't handle the inrush. I can't remember the details but I know that there are some builders who have never installed them and other who do so only because the peoples demand it. As far as I know, modern caps are solid enough that even the cheap ones are more than potent enough.
I also believe that it's good practice in some high frequency applications... but guitar amps don't play in that range.

This project isn't moving fast... but it is moving.
Just received the delivery from Tone Tubes. Most of the more conventional electronics should be here next week.