Identifying Power Transformer leads for FENDER Amp

[nickb]

Thanks you guys! I've always heard [read on forums] that that .01 cap increases bass as you go bigger. Some recommend .022 but say .047 can add too much bass. Is this completely backwards?
Most people jumper that .0047 but I tried that with my other MMB amp and found it added bass, so I like that cap there. I'll try a .0022!

You are correct, it was just a dumbo typo on my side - all those 00000's. I meant 0.0047uF.

I usually say "1nF" in preference to 0.001uF as it less confusing but was trying to be helpful be being consistent with what was on the dwg.

I might add, I deliberately didn't mention the bright cap ( that you like) as it's purpose is a bit different:
(1) At low volumes, it stop the loss of highs due to the input capacitance of the following stage
(2) It compensates for the natural loss of sensitivity of the ear at low volumes
(3) To a limited extent, it duplicates the tone control. i.e When you turn the tone up it bypasses the bright cap.
(4) When you turn the volume up it bypasses the bright cap. But if you like it at low volumes you'll probably still like it a higher ones bearing in mind the above.

[czech-one-2]

Tried the .0022 and combined with the bright cap this thing sounds nice,chimey and bright. Very 'silverface' Fender mini-Twin. Really couldn't be much happier.
You guys that helped should be commended for taking your time to help folks out. I've been here 3 different times now and each time you all have helped me successfully resolve whatever electronic problem I had. I'm a slow learner at this stuff but getting more knowledgeable each time I fire up the soldering iron or visit here.
Come to Prague and beers are on me!

Oh, and if I can find a cabinet builder to make a blackface champ head this amp will be my grab and go-to!
Thanks Enzo,Jmaf nickb and g1!

[czech-one-2]

One more question regarding this amp. I'm considering putting in a vibrolux OT so that the 8ohm speaker load is a better match and clean headroom is maximized. Any foreseeable problem with running this OT in my amp? It currently has a Fender [hammond] Deluxe OT.
Deluxe OT 8500ohm

Vibrolux OT 4000ohm

[g1]

That Vibrolux OT you are showing has a secondary @ 4 ohms.
So if you put an 8 ohm load on it, it will have a primary of about 8k, almost the same as the Deluxe OT.
What is your desired primary impedance?

[czech-one-2]

Actually, I'm using an 8ohm speaker. But was told [read] that by changing from 6v6's to 6l6's the amp wants to see a 4ohm load.
So I'm a bit perplexed...

[g1]

Probably what was meant was that if you load the Deluxe OT with a 4 ohm speaker, it will be the right match for 6L6 usage.
This is easier than changing the OT.

[jmaf]

Yeah 6L6's would naturally lower the impedance on the primary. But it's a self biased output stage which means the tube is self regulating, so messing with the output impedance in the 4 to 8 ohm range won't do any harm to the 6L6's. Just listen for tone changes - you may or may not like the tone of a slightly mismatched impedance. When impedances are badly mismatched you usually lose tone in a significant way and you can hear it. OTOH sometimes a slightly mismatched impedance is key to a great tone. You can experiment at will, you won't harm that self biased stage.

[Steve A.]

The only way to really get more headroom would be to use a 4 ohm speaker instead of 8. That will double the output power assuming the driver is up to the job...

I know that is true for solid state amps but I hadn't noticed a big difference like that with tube amps...

Steve Ahola

[czech-one-2]

Well, 4ohm speakers aren't an option as I'm quite fond of the tone of my 8ohm McKenzie 12'' and oxford 4x10''. The amp sounds great but maybe a small amount of not so great break up when its cranked [which is very loud!] I thought the vibrolux OT would be better suited for this amp but would need something with an 8ohm secondary.
I'm ok with it as is but at this point want to do everything possible to set this up as cohesively and ideally as possible. Its going to be a great little amp!
I think next I will take nickb's advice of swapping the 10k for 33k to not drive the 12ax7 so hard.

[czech-one-2]

Wow you guys, just played it a couple hours and it really needs nothing at this point. Did the 10k >33k swap as well as lowering the cathode resistor to 580ohm 10w and it's as loud as any deluxe I've ever heard and clean all the way up the dial! Not sure what impedance its putting out but the tone that I was trying to achieve is there.It's a wrap!!!

[Steve A.]

For future reference this appears to be the correct schematic for this model...

Prowess Amplifiers - Fender - Schematics - Musicmaster Bass

musicmaster_bass_schem.pdf

Steve Ahola

[czech-one-2]

^ Great! If anyone sees something they would tweek for safety/reliability with this amp now at 480v feel free to comment!
It's running cool and quiet though, no fuse tripping,hum,excessive heat or other issues.

[czech-one-2]

Heater voltage?

Ok, another question relating to this amp.
Why is my heater voltage on pins 2/7 44.4v? Shouldn't 6.3v be correct?

[Enzo]

Not quite, you should have 6vAC BETWEEN pins 2 and 7, not to ground.

The original schematic shows one side grounded, so you would in this case measure zero volts on pin 2 or 7 and 6v on the other. However, someone may have elevated teh heaters by some DC voltage. You have not specified if your 44v is AC or DC.

[czech-one-2]

Yep, AC reading is 3.2, 42vdc...
But shouldn't the AC reading be 6.3v?