Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on January 03, 2021, 03:34:45 pm

Title: Voice of music model 136
Post by: goldstache on January 03, 2021, 03:34:45 pm

I love this amp.
Sounds great, little bass heavy depending on cabinet. Will throw a bright cap on the volume up front and see if it regains something presence when set low.


Here’s my question though.
The NFB network also feeds to the ring of the TRS mic input. If I’m just using a mono cable, doesn’t that just put ground at the junction of the bulk NFB resistor and the filter that feeds v1b cathode???


If it were actually a balanced feed this may be where the (-) signal inserts near unity to appear on V1b plate and onward. Like a cheap in phase mixer????


Thoughts on the circuit when ring grounded (mono guitar) and when it’s used as designed, ha!, balanced signal in.


Thanks,


Mock one up, it’s one of the few 2 band equipped 2 bottle amps I’ve got around.


Have not tested the OT for specs yet. It has “606” stamp so Wood/schu.


Any guesses?? I feel like it’s part of the magic for sure.

Title: Re: Voice of music model 136
Post by: PRR on January 03, 2021, 06:16:42 pm

It is a cheezy trick to raise Mike gain about 15X over Phono gain by shorting-out the output stage NFB.

Not "balanced".

Title: Re: Voice of music model 136
Post by: goldstache on January 03, 2021, 08:26:20 pm

PRR, thanks!
Makes sense to me now. You should be in the flashlight biz, the amount of things you’ve illuminated for me over the years!


As for OT impedance. Probably 10k or so, don’t feel like testing for turns. Somebody just falsely confirm so I can avoid dragging out the test jig.

Title: Re: Voice of music model 136
Post by: DummyLoad on January 03, 2021, 09:59:21 pm

to shave some lows, try replacing the coupling cap of the .047uF/470KΩ of the 6BQ5 to .0047uF or .01uF. using a .01uF rolls off at ≈33Hz, a .0047uF rolls off at ≈72Hz. 

--pete

Title: Re: Voice of music model 136
Post by: goldstache on January 03, 2021, 10:59:13 pm

Yeah, that’s what I’m thinking too. Sounds good all the way up, and the big cap there sets it right at the beginning of some time constant issues. 8v on cathode 272v across 6bq5. I’ll try the the treble bleed across input volume to start.


Thanks for the cutoff info!



Should be about 40ma at that plate v for 90%. This thing is pulling 48ma or so!!


Sounds good, ain’t on fire

Title: Re: Voice of music model 136
Post by: PRR on January 04, 2021, 10:57:48 pm

> OT impedance.

Allowing for higher line voltage, it is almost spot-on the datasheet numbers.
https://frank.pocnet.net/sheets/049/6/6BQ5.pdf
4k-5k?

Title: Re: Voice of music model 136
Post by: goldstache on January 08, 2021, 12:53:52 am

Yep it showed turns hinting of 5k:8 after the numbers were crunched.


Thanks everybody.

Title: Re: Voice of music model 136
Post by: goldstache on January 20, 2021, 07:26:13 pm

Liked it so much cloned it out parallel everything. Stayed SE.
Here’s some fraudulent pics.