Amp gurus , PT questions and others...

[StrummersOfThunder]

Amp gurus I call on you again !

Im deciphering a (reportedly ab165) blackface bassman clone. Its about 15 years old, I picked it up in Aussie and later sold to a band mate.
I really like how it sounds. Its loud as balls but has a great compression to it that Is different to other blackfaces Ive owned.

Ive ordered small parts and chassis etc to try and recreate it (decided this was a more achievable build that the harmonic trem brownface bassman and I allready have a brown voiced tremolux).

Few things Ive noticed:

1. Its missing V1
2. Its variable biased (cathode biased) rather than equipped with a bias trim (or maybe more accurately a hum pot) like the fender schematic calls for. I note a large 330hm resistor and a 220mf/100v cap. Im assuming these are the bias cap and resistor
3. The power transformer is a mojotone blackface bassman. Its secondary has 358v-0-358c (716 VAC) at 250ma. The fender and hammond repro codes call for 330-0-330 (660v VAC) at 275ma.
4. There is a ground lift network that looks familiar to the valve wizard diagrams that Ive used for an outboard reverb tank exceopt for that there is no 5w10r resistor, just a 100nf cap.

From what I read, the conversion to cathode bias is actually well thought of with big iron stuff (depending on author). Can take away a bit of that fatiguing 'attack' and lack of compression.

Im aware that pulling V1 is a trick to increase gain to the normal channel but ive not seen an amp where the wiring is taken out. In this one both channels still operate.


My questions are:
Should I worry about the PT? ie just get the locally available hammond. Will that throw out things or will the slightly higher current and lower voltage of the hammond equal out the higher voltage and lower current of the mojo? am I right in saying the voltage supplied will be fixed and the current rating is just the amount that the tranfomer can deliver safely without it being stressed out? So should be no harm in going with a higher current rated transfomer so long as the valves and circuit can handle to voltage?
Spec sheets for both:
mojo
https://www.mojotone.com/Blackface-Bass ... nK00Fhp0rB
hammond
https://www.hammfg.com/files/parts/pdf/ ... 1697661948

There are quite a few hammond versions that will supply 710vac but with 300ma current. I guess im wondering if I go with something like that to matchwhats in the amp im looking at or I stick with the fender replacement.

At either rate will I guess Ill need to do some measurement and work out the correct resistor value for the bias circuit.

The amp im looking at sounds so goo dthat I just want to clone it as close to possible at first pass, then maybe look at mods such as MV and mid tone pot etc down the track.

Here are a couple of pics of what im looking at.

[StrummersOfThunder]

[StrummersOfThunder]

[StrummersOfThunder]

[MikeC]

Draw a schematic and clone it.

[StrummersOfThunder]

Draw a schematic and clone it.

wizdom

[StrummersOfThunder]

Draw a schematic and clone it.

So im guressing that means forage thru the hammond listings for a PT that closely matches?
i.e a bandmaster or super reverb export version would get 710vac for 300ma, as vs the one in the pictures which supposedly has 716VAc 250ma?
I could get a mojo one but its deadly expensive to ship.
I do have a possible work around with a friend coming back from the states next month.

[StrummersOfThunder]

I’ve drawn a layout with all the components. I have to say I’m still a bit intimidated by schematics but maybe it’s time to conquer that and do a bit of a deeper dive.

[jeremyb]

Toan is in the black tape over the B of Bassman

Do you have any details still of the person you bought it from? it would be cool to know more about this amp!

[MikeC]

Given that the amp sounds so good to your ear I reckon match the PT too. Then there's no doubt you'll get close with your build.

[StrummersOfThunder]

Toan is in the black tape over the B of Bassman

Do you have any details still of the person you bought it from? it would be cool to know more about this amp!

HEY !!! Assman!!!

If you know you know ….

Nah I picked it off gumtree in Aussie in 2016. The tape over the B was an after market mod that we added when Justin started using it live for pedal steel in the band

[StrummersOfThunder]

Given that the amp sounds so good to your ear I reckon match the PT too. Then there's no doubt you'll get close with your build.

Do you see any issue with a Hammond 710vac 300ma in place of the mojo 716vac 250ma ?

[MikeC]

One issue, the power tubes cathode resistor value will probably need changing if you use a different PT. Also, a different PT will behave differently (voltage sag wise) to the Mojo.

[StrummersOfThunder]

One issue, the power tubes cathode resistor value will probably need changing if you use a different PT. Also, a different PT will behave differently (voltage sag wise) to the Mojo.

I’ll draw the schematic and measure some voltages

[StrummersOfThunder]

My guess is mojo made one transformer to cover a range of kits and altered their circuits around it