Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RoadShow on April 12, 2026, 12:25:21 pm

Title: Bias Reader Built, works great, and revealed an issue.
Post by: RoadShow on April 12, 2026, 12:25:21 pm

I went with the Tube Depot solution because it also allowed reading the plate voltage, and that's essential for targeting the bias point level.
Instead of building as shown in their documentation, I decided to mount 4 pods to a box, run the bias voltages and plate voltages to a break before make switch and run the selection to a red banana jack and pass the ground through to a black banana jack, and run banana plug to plug cables to my meter.  Between pods, box, switch, and jacks I have about $100 in to it.

See pic.

I tested it on a recently purchased Fender TRRI in super clean condition that was at a super price that seemed to play well.
However, I was getting no current reading out of V7 and V10 6L6GC tubes.  I swapped pods with reading with pods that had no reading and the issue stayed with the tube.  So, I slid the chassis out and a quick inspection and a meter showed that the 470ohm screen resistors for V7 & V10 were open.  They didn't look burned at all, but read open.  I replaced them with fresh ones and was back in business.

With the box I read the plate voltage and set the bias to 60% based on that.

So, I consider it good timing that I built this tester, who knows how long that amp would have been running on two tubes!!

Title: Re: Bias Reader Built, works great, and revealed an issue.
Post by: stratomaster on April 12, 2026, 01:07:03 pm

Doing an output power test as part of servicing is a good practice to to develop, as is a basic voltage check on all the pins of the power tubes.

Both of these would have caught the reduced capacity of the amp without the need for a complex bias device. 

Glad it helped you, but don't shortchange your troubleshooting/maintenance practices in favor of a device.

Title: Re: Bias Reader Built, works great, and revealed an issue.
Post by: shooter on April 12, 2026, 02:21:26 pm

Quote

but don't shortchange your troubleshooting/maintenance practices in favor of a device.

you win the Quote of the 21st century!!!   :occasion14:


the masses have relegated "practices" (understanding n knowledge) to devices

Title: Re: Bias Reader Built, works great, and revealed an issue.
Post by: acheld on April 13, 2026, 09:54:21 am

"But, we have AI now . . ." 

Quote from a 19yo, a good kid otherwise.    :icon_biggrin: