Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AHeck on August 19, 2020, 06:10:13 pm

Title: AA165 Now With Four 6L6s, and low output.
Post by: AHeck on August 19, 2020, 06:10:13 pm

Good evening, I have been fussing over a new build, an AA165 Bassman that I built from scratch with Twin Reverb transformers to support the extra 6L6GCs wired in parallel and a standard pre-amp section.  Voltages are within 25 VDC all around, no real cries for help, it's got a very quiet noise floor, so no real cause for alarm.  Used the Classic Tone 40-18113 Output Transformer and I had a four ohm cabinet run into the four ohm taped output, as I thought made sense, but I was not getting and volume and the tone was monstrously distorted.  Retested a bunch of stuff and finally ended up seeing that removing two power tubes requires a change of impedance, so as I had added, I switched the load to the eight ohm out and the amp sounds glorious.  Now, to help in the future, in the design process and such, what happened? Please, I really need to know.  I can't just be happy it works,. I've been puzzling turn ratios and trying to find output impedance for four 6L6s and other such mess.  can anyone point me right? 

Title: Re: AA165 Now With Four 6L6s, and low output.
Post by: shooter on August 19, 2020, 06:55:31 pm

Quote

wired in parallel

if so, when it's off n safe, put the "other 2 in" and take the "glorious" ones out, retest?

Title: Re: AA165 Now With Four 6L6s, and low output.
Post by: ac427v on August 19, 2020, 07:47:18 pm

Sounds like one of the tubes or sockets are bad. I like Shooter's idea of swapping the tube pairs and seeing if it takes away the glorious.

Title: Re: AA165 Now With Four 6L6s, and low output.
Post by: AHeck on August 19, 2020, 09:07:53 pm

       I'm very sorry, I must have been unclear, i haven't removed any tubes.  All I did, at all, was to switch the speaker cable from the 4 ohm output to the 8 ohm output, and instant "gloriosity". Crystal clear tone and volume for days.
       I had come across the impedance matching concept in an article about power output modification by removing two of the tubes in a quad, but I didn't do that.  Technically, I added two tubes to the original AA165 circuit.  As to performance, the problem is solved, barring a cataclysmic failure by the transformer from running a 4 ohm load from an 8 ohm tap. Now I need the whys.

Title: Re: AA165 Now With Four 6L6s, and low output.
Post by: sluckey on August 19, 2020, 09:34:39 pm

Look closely at your speaker jacks. Be sure that neither jack is a switching type with the switch lug connected to ground.

Title: Re: AA165 Now With Four 6L6s, and low output.
Post by: pdf64 on August 20, 2020, 03:00:11 am

Yes, those symptoms align with the 8 ohm jack being wired with a shorting contact.

Title: Re: AA165 Now With Four 6L6s, and low output.
Post by: AHeck on August 20, 2020, 04:41:01 am

YUP.  They do indeed.  Thanks.