Amp gurus , PT questions and others...

[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.

I’ll draw the schematic and measure some voltages

Good idea. I would calculate where the current power tubes are biased at in terms of % of max plate dissipation. Then you can "match" this bias point with a different PT/different cathode resistor. It's often around 100% but you won't know on this amp until you calculate it.

[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

Good idea. I would calculate where the current power tubes are biased at in terms of % of max plate dissipation. Then you can "match" this bias point with a different PT/different cathode resistor. It's often around 100% but you won't know on this amp until you calculate it.

Perfect
I need to look at the ab165 schematic and see where the voltages need to be recorded

[Optical]

Yeah what Mike has said.
Generally speaking, I'd try to match the power transformer if power amp distortion/response/sag is important to match or makes the amp what it is. If it's not an amp that relies on that (i.e. mostly high gain metal amps) then a close match of the power transformer isn't particularly important.

[jeremyb]

Hmmm images have gone, its not one of these is it?

https://www.mojotone.com/Amp_Kits/Brown ... man_WD.pdf

https://www.mojotone.com/Blond-Bassman-Amplifier-Kit

[StrummersOfThunder]

Hmmm images have gone, its not one of these is it?

https://www.mojotone.com/Amp_Kits/Brown ... man_WD.pdf

https://www.mojotone.com/Blond-Bassman-Amplifier-Kit

Nope but it does have a presence pot on the back

[StrummersOfThunder]

Yeah what Mike has said.
Generally speaking, I'd try to match the power transformer if power amp distortion/response/sag is important to match or makes the amp what it is. If it's not an amp that relies on that (i.e. mostly high gain metal amps) then a close match of the power transformer isn't particularly important.

Im not pushing it into power amp distortion no
But there is a certain compression it achieves that other big iron fenders I’ve had don’t

[Optical]

Compression is distortion. So it sounds as though your power amp signal will either be hitting the power rails from amplitude, possibly also from power supply sag. So getting that compression will be partly due to the power transformer - the voltage it outputs determining where the supply voltage is, and how quickly the power supply can recover based on the available supply current - sag.

[StrummersOfThunder]

Compression is distortion. So it sounds as though your power amp signal will either be hitting the power rails from amplitude, possibly also from power supply sag. So getting that compression will be partly due to the power transformer - the voltage it outputs determining where the supply voltage is, and how quickly the power supply can recover based on the available supply current - sag.

Very nice description , I think you've nailed it.
Sorry if I come across a bit slow with all this. Ive been interested in amps for ages but have taken short cuts in understanding the deeper processes going on and just painted by numbers as it were. Pretty keen to develop at least a basic idea around these circuits and what makes them do what they do.

[MikeC]

Compression is distortion. So it sounds as though your power amp signal will either be hitting the power rails from amplitude, possibly also from power supply sag. So getting that compression will be partly due to the power transformer - the voltage it outputs determining where the supply voltage is, and how quickly the power supply can recover based on the available supply current - sag.

Agreed - this kind of "amp compression" is a form of distortion and occurs when you try to force the output power beyond the power supply's capability. The PSU voltage starts to drop as the maximum available current is exceeded.
Tube amps do this more "warmly/musically" than typical solid state amps.

As opposed to the compression from audio "compressors" which deliberately flatten the dynamic range of the overall output by amplifying the lower amplitude signals & attenuating the higher amplitude signals all without distorting any of the original signal waveforms!

[StrummersOfThunder]

Compression is distortion. So it sounds as though your power amp signal will either be hitting the power rails from amplitude, possibly also from power supply sag. So getting that compression will be partly due to the power transformer - the voltage it outputs determining where the supply voltage is, and how quickly the power supply can recover based on the available supply current - sag.

Agreed - this kind of "amp compression" is a form of distortion and occurs when you try to force the output power beyond the power supply's capability. The PSU voltage starts to drop as the maximum available current is exceeded.
Tube amps do this more "warmly/musically" than typical solid state amps.

As opposed to the compression from audio "compressors" which deliberately flatten the dynamic range of the overall output by amplifying the lower amplitude signals & attenuating the higher amplitude signals all without distorting any of the original signal waveforms!

I think maybe im over stating the compression thing. Its hard to tell though. What im describing it a clean amp, but with something that sets it apart from other big iron amps. When ive played showmans, and some other bassman amps, and even bandmasters, ive found this jack hammer like attack that makes it kind of jarring to play. When i played this the other night I barely turned the volume up past 2/10 maybe 3/10 (then the soundy came and asked me to dial it back). So its not like i was cranking it and putting the transformer through its paces. But it was definately different to a crystal clean twin/DR as well. Maybe a touch more mid forward. I'll tell you what it didnt have was the shrill glassy thing that doesnt play well with an outboard tank, even with the bright switch on. Just an all round well balanced amp for surf/single coil stuff and with plenty of 'cut throught the mix'.

Anyhow I dont think ive got a world of choice in the hammond 290 range. 710VAC and 300ma seems to be the go. Or its 660 at 275 or the mojo at great expense with 716VAC and 250ma.

[StrummersOfThunder]

Any reason fender seems to have some amps with a 50v bias tap on the PT and some with 53v??

[Jay]

50V equals 50V

[Jay]

The photos have disappeared so cannot see if it has a valve rectifier. I presume with the PT specs you have provided, the amp has a valve rectifier. The PT and rectifier valve chosen determine some of the amp's characteristics.

[StrummersOfThunder]

The photos have disappeared so cannot see if it has a valve rectifier. I presume with the PT specs you have provided, the amp has a valve rectifier. The PT and rectifier valve chosen determine some of the amp's characteristics.

Solid state

[Bg]

Looking for the photos too, on imgur maybe not set for sharing or something?