Marshall 10 watt combo clone

[Diabolical Artificer]

Sorry Rob, havn't forgotten about your tfmr, well I had, but just remembered again, it's just been a hectic week. please send me a PM if I forget again.

If you get some F&T capacitors they're rated for 500v, just the job for your amplifier smoothing/resevoir caps. Watford Valves sell em as does HiFi collective.

Like resistors it's what the cap see's in that part of the circuit its in. So for a cathode R, you only need a 2v rated cap. It's usual to use 25v caps. But for a coupling cap say, it's usual to use a 630v poly cap.

I used to get confused about this too, your not being a dunce.

Andy.

[Bushbaby]

Thanks Andy
I've some vintage Mullard mustard, red Wima polyprops , and various other vintage caps of varying materials. I've a second JJ smoothing cap that I bought at the same time as when I was working on the Bronco. Next job is to prepare the turret board with tags (I've a couple of old phenolic boards so I was going to use one of these) so I have the horrible task of drilling this to suit (plenty of extraction).

Regards

[Bushbaby]

All,
I hope you are all keeping well.
I am now determined to get on with this project having virtually all components, case & chassis, Transformers (thanks to Ed Dining) etc.
There is a modification that I want to incorporate to the circuit which was not included (due to original costs of the unit) which is to add a second ECC83 to the input so that the major issue described as follows (taken from Guitar.com):

The issue is that the volume and tone controls are situated between the screen grid resistors and the grid of valve. What’s more, the two channels are joined by a couple of 470k mixer resistors, which attenuate the guitar signal before it even reaches the valve. The volume and tone controls of valve amps – including most Marshalls – should come after the first valve stage.

I've added the relevant part from the modification that was undertaken (screenshot attached     ) and another copy of the original circuit diagram      . I've highlighted in yellow an area I'm having a problem with.

Can anyone please offer help/advice so that I can get started soon.

Thanks in advance

Rob

[Crackle]

The issue is that the volume and tone controls are situated between the screen grid resistors and the grid of valve. What’s more, the two channels are joined by a couple of 470k mixer resistors, which attenuate the guitar signal before it even reaches the valve. The volume and tone controls of valve amps – including most Marshalls – should come after the first valve stage.

I don't understand that statement about the screen grid and grid of the valves, are you sure they are not talking about another amp with a modified circuit.

If you modify the circuit by adding a preamp then it will never be a Marshall 10 watt clone and you will always wonder what one sounded like.

Mike

[Bushbaby]

Hi Mike,
Thanks for this.
I am just concerned about reducing noise/hum. If this can be done without major modification then I will be glad to do so.
I believe that there is an issue with the tremolo circuit that was wrong from day one of manufacture that just requires a minor tweak in the circuit (not a major modification) that I will be happy to do.

Best Regards

Rob

[Bushbaby]

This is the tremolo modification from the 1930 amp design to the later 18 watt models

   

[boater sam]

I see a standby switch on the 1930 amp circuit, cutting all the HT off.

This is a bad idea, you will get cathode poisoning if the HT is completely cut off, aim to maintain a few mA of conduction on every valve.

[Bushbaby]

Please, I am not moaning and its good to see I've had near 5000 views (thank you all) but only 36 replies (0.7% response)....I will move on and do the best I can.

Regards

[ppppenguin]

I reckon 36 replies is a good tally. Many (most?) threads don't even get to double digits.

[Bushbaby]

Hi Jeffrey
Again, it wasn't a moan - I was just asking for some guidance at the end of the day. Its not a highlight at the moment as due to the snow and cold weather I can't get out to the workshop (still feeling poorly after having Covid and then my Covid 1st jab!)

Best Regards