Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jcm-jmp on April 03, 2010, 02:45:33 pm

Title: 6g6 blonde bassman
Post by: jcm-jmp on April 03, 2010, 02:45:33 pm: hi all,

i have a 6g6 blonde bassman in for repair that when volume is low sound great, but when pushed into overdrive it seems like it sags to the point of almost cut off. very slow recovery and very un musical distortion. have replaced rectifier with 5ar4 and a 5u4 same results. disconnected the choke and installed a 5w 470 ohm ressitor still no change. all caps b+ and signal chain have been replaced. bias circuit has been reworked to match dougs conversion for fenders. Im lost and dont know where to look from here.

thanks
robert
Title: Re: 6g6 blonde bassman
Post by: jjasilli on April 03, 2010, 06:59:04 pm: http://geofex.com/ > -Tube Amp Debug Page > Low Power or Loss of Volume
Title: Re: 6g6 blonde bassman
Post by: New2ampsman on April 04, 2010, 09:48:35 am: I'm new to this stuff and know virtually nothing about amp debugging from a "getting my hands dirty point of view", but from all the books I've read, could this be due to a parasitic oscilllation at such high frequency that it's causing a cut off?
:undecided:
Title: Re: 6g6 blonde bassman
Post by: tubeswell on April 04, 2010, 01:24:27 pm: What are your voltages?
Title: Re: 6g6 blonde bassman
Post by: HotBluePlates on April 04, 2010, 03:10:52 pm: Quote from: tubeswell on April 04, 2010, 01:24:27 pm
What are your voltages?

Especially your bias voltage. And exactly how big is the signal when the gross distortion happens.

I'm just wondering if there's too little bias voltage -> too much plate current, and then incoming signal is radically bigger than the bias voltage. If so, you can get nasty grid-blocking that can sound rough and take some time to recover.

Also just wondering if the amp sounds fine just "a little distorted" and you're slamming the output tubes way beyond that point.
Title: Re: 6g6 blonde bassman
Post by: jcm-jmp on April 04, 2010, 06:58:56 pm: I'm just wondering if there's too little bias voltage -> too much plate current, and then incoming signal is radically bigger than the bias voltage

dead on HBP!

i re-biased, which its a tad hot, but sounds great!

now the only problem is bass pot has no effect at all on the bass channel. i have replaced all componants associated with it but still nothing?

i do have bass in the signal just no control.

thanks guys
robert
Title: Re: 6g6 blonde bassman
Post by: jcm-jmp on April 04, 2010, 09:50:23 pm: just found this post and cleared up my ignorence!

Re: 45548 Twin OT in 6G6 Bassman
« Reply #8 on: February 10, 2010, 05:40:30 pm » Quote

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If the presence pot is 25K linear, shinted across 4700 fixed, it just aint gonna do much until it is 15% from the end. If slamming it to one end IS different from the other end, that's how it was designed. There's other ways to do this, but not "authentic".

> The bass control in the bass channel

Sorry, I didn't realized how twisted the 6G6 bass channel was. The caps are 0.25u, and the bass-to-ground resistance should be 820 ohms! The 250K bass pot against the 1Meg "slope" resistor means bass-knob action is small, and entirely below 100Hz (negligible action on guitar, only bottom-octave of bass).