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GrantB wrote:I have always wanted a Harvard!

My vibrolux is basically a harvard with trem....

That’s why I like

jvpp wrote: C5 - I have chosen a value in between what you suggested, so going with 120pF.

With R7 clarified, the tone controls are actually a bog standard Fender tone stack, but the mid pot is replaced with that fixed resistor. You could put a 25k pot in place of that resistor, even internally, to see if something better emerges.

Also the C3 4.7nF cap is typically 47nF in a Fender, so check it has been read correctly too.

Anyway, the circuit isnt particularly sensitive to the C5 treble cap value, but you could experiment with the value from 100pF to 1nF. Or model it using the handy Duncan tone stack calculator program

This is starting to feel like watching exploratory surgery on my kid.

Optical wrote:

jvpp wrote: C5 - I have chosen a value in between what you suggested, so going with 120pF.

With R7 clarified, the tone controls are actually a bog standard Fender tone stack, but the mid pot is replaced with that fixed resistor. You could put a 25k pot in place of that resistor, even internally, to see if something better emerges.

Also the C3 4.7nF cap is typically 47nF in a Fender, so check it has been read correctly too.

Anyway, the circuit isnt particularly sensitive to the C5 treble cap value, but you could experiment with the value from 100pF to 1nF. Or model it using the handy Duncan tone stack calculator program

Thanks, will do. Which fixed resistor you are referring to? Confirming C3 is 4.7nF.

Now the trannies...

What you folks recommend and where to buy them?

Slowy wrote:
One final point: Don't think for a second that I don't miss it!

I think we now have a good circuit diagram. I have plenty of photos of its guts and taken chassis measurements, so should be all good to swap our amps back again.

Preparing the Bill of Materials now and just need transformer data to complete. I will then post the BOM.

jvpp wrote: Thanks, will do. Which fixed resistor you are referring to? Confirming C3 is 4.7nF.

R7 - the 6k9. It's a strange value which I think would only be explained by the Jansen designer having a pot there, turning it to where they liked it best, then replacing it with a fixed resistor. I'm just suggesting you could use a pot instead of R7 which would provide a mid control and of course the factory setting when in the right place.

For the power transformer, just about anything which produces the right output voltage will be fine (BTW, did you measure the output voltage?). Hammond transformers are free delivery from Digikey. The output transformer is more tricky though, you will want to closely match the input impedance and also the core specs. The input impedance will affect the tone and the core size will affect how quickly it saturates. If i was going to guesstimate something similar, i would try and find an OPT about the same weight; but I would make the effort to measure the input impedance. Or just skip all the hassle and get some old 15W Fender spec one.

I have a gunshot of one of these but it is too large a file to post here.
The OP transformer has 4, 8, 16 Ohm taps marked and the image is good.
PM me your email for a copy.

The output transformer measures 300 ohm on the primary (in circuit), no power applied. Is that what one should expect?

Courtesy of Alan (Anvil Amps) we now have two chassis photos of two different 4/12 amps. Good consistency between them in respect of parts used. I note one extra capacitor in the tone stack and the pipe capacitor has changed into something black of which I cannot read the value.

Top - Alan's
Bottom - Slowy's

Yep, it doesn't mean much though (except its not burnt out), you would normally measure the impedance at 1Khz, usually about 8Kohms to 10Kohms for an EL 84 transformer.

sizzlingbadger wrote:Yep, it doesn't mean much though (except its not burnt out), you would normally measure the impedance at 1Khz, usually about 8Kohms to 10Kohms for an EL 84 transformer.

Great, thinking this one might do?
https://www.amplifiedparts.com/products ... der-15w-8k

Only problem is that it is a single secondary. Would prefer the 4 & 16 ohm combinations

Looks good to me

jvpp wrote:

sizzlingbadger wrote:Yep, it doesn't mean much though (except its not burnt out), you would normally measure the impedance at 1Khz, usually about 8Kohms to 10Kohms for an EL 84 transformer.

Great, thinking this one might do?
https://www.amplifiedparts.com/products ... der-15w-8k

Only problem is that it is a single secondary. Would prefer the 4 & 16 ohm combinations

Get the 1760E instead of the 1750E

jvpp wrote: something black of which I cannot read the value.

It's a "silver mica" type of capacitor. Pricey but potentially more stable alternative to a ceramic cap when small values are wanted e.g 10pF to 10nF.
Maybe look at picking up a cheap LCR meter from ebay (or borrowing one?) to measure the cap values? ...Or just use your ear.

Optical wrote:

jvpp wrote: something black of which I cannot read the value.

It's a "silver mica" type of capacitor. Pricey but potentially more stable alternative to a ceramic cap when small values are wanted e.g 10pF to 10nF.
Maybe look at picking up a cheap LCR meter from ebay (or borrowing one?) to measure the cap values? ...Or just use your ear.

Silver Mica I think it was 250pF