Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Stuff4bikes on July 06, 2021, 01:27:55 am

Title: Bartell/hohner amp, bias question
Post by: Stuff4bikes on July 06, 2021, 01:27:55 am: Anyone familiar with these? All I have is the chassis, no enclosure or speaker.

Can someone explain the bias method on this amp...I am fairly new to this stuff but I cant remember seeing this method.
Title: Re: Picked up a bartell contessa ca 15
Post by: Latole on July 06, 2021, 02:45:54 am: https://bartell.vintageusaguitars.com/2021/05/19/ca-15-mini-bass/

Google ; " honer contessas CA 15 amp" and you'll many informations

What are you looking for ? Is amp working or not ?
You want to add a speaker ? Any 8 ohms 20 watts (or more) speaker will work .
Whant to built cab ?
Title: Re: Picked up a bartell contessa ca 15
Post by: Stuff4bikes on July 06, 2021, 07:43:49 pm: Just some random question.....

It says bass amp....how does a guitar sound through it.....
How did the chassis mount originally.
I have noticed different variations online. Mine has a toggle and fuse on the front right side. Mine doesn't say hohner on the front.
Some had a 12" speaker or a 15"
They made a guitar and bass CA-15, it sounds like.....what's the difference....
Title: Re: Bartell/hohner amp, bias question
Post by: Stuff4bikes on July 07, 2021, 09:07:29 pm: I put a 2amp fuse in, powered it up with my dim bulb and it sounds pretty good. No hum that I hear.....everything looks factory except it looks like cap can was changed....
Title: Re: Bartell/hohner amp, bias question
Post by: st on July 08, 2021, 12:42:21 am: Fixed bias, it seems. But something's wrong in the supply on the schematic. Who drew that? Pics of actual circuitry?
Title: Re: Bartell/hohner amp, bias question
Post by: Stuff4bikes on July 08, 2021, 02:15:40 am: Quote from: st on July 08, 2021, 12:42:21 am
Fixed bias, it seems. But something's wrong in the supply on the schematic. Who drew that? Pics of actual circuitry?

I got it from a bartell fan site. Its stated he owns this amp and several others and he drew/put the schematics on line.....
I will get some pics tomorrow of the circuitry.
Title: Re: Bartell/hohner amp, bias question
Post by: Latole on July 08, 2021, 03:17:53 am: Quote from: st on July 08, 2021, 12:42:21 am
Fixed bias, it seems. But something's wrong in the supply on the schematic. Who drew that? Pics of actual circuitry?

It is a fixed bias.
AC voltage for bias circuit is wrong;
What it should be ;
Title: Re: Bartell/hohner amp, bias question
Post by: Latole on July 08, 2021, 03:34:17 am: Two modifications I would make would be to rewire the heater.
Unplug the ground side of the transformer and the heater side to install a wire. The two wires feeding the heaters should be twisted together.

I would add an artificial ground with two 100 to 220 ohms resistors. ( screen shot ) ; http://www.valvewizard.co.uk/heater.html

And I'll add 3 prong grounded power cord.

This modification would make the amp quieter and safer.
Title: Re: Bartell/hohner amp, bias question
Post by: PRR on July 08, 2021, 11:50:22 am: Pin 8 V2 is surely higher than 80V, more near 100V-110V DC. I dunno if the parts can drift that much or just a typo.