Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: etliharr40 on May 08, 2022, 03:32:05 pm

Title: Hoffman Blues Junior with No master Volume and No Meat
Post by: etliharr40 on May 08, 2022, 03:32:05 pm
About to start building another Hoffman blues junior board. I have power and output transformers from an old blues junior and am planning to put the build in the chassis of an 80s Fender "75" tube amp. I'll utilize the five 9 pin tube sockets/holes already in the chassis. I'm wanting to try building this and omitting the master volume and the meat control. Wondering if this is as simple as going straight from the .001 cap and 430k resistor to the 220k resistor (bypassing the master potentiometer)? as for the "meat control", what parts would I ommit/bypass to remove this from the circuit without effecting everything else?
Title: Re: Hoffman Blues Junior with No master Volume and No Meat
Post by: sluckey on May 08, 2022, 03:59:14 pm
Refer to Hoffman's schematic...

Quote
Wondering if this is as simple as going straight from the .001 cap and 430k resistor to the 220k resistor (bypassing the master potentiometer)?
Yes, but you also need to connect a 47K resistor from the junction of R18 and R20 to ground.

No Meat... Remove R60 and C40. Connect R27 directly to R26.
Title: Re: Hoffman Blues Junior with No master Volume and No Meat
Post by: etliharr40 on May 16, 2022, 03:08:18 pm
finished my build and fired up the amp. Most of my measurements look pretty close to the schematic. B+ = 344V,Z=332V, Y=284V, X=257V. Pin 7 of EL84s approx 347V, Pin 9 of EL84s 336V, C- = -12, pin 3 of EL84s approx 33mV. My output seems to be lacking a lot of power (turned up to 10  and sounds very thin and harsh and not anywhere close to the loudness of a blues JR turned all the way up.) Power tubes are new and I'm waiting on a new set of preamp tubes to rule out those (although by the sounds coming out I'm guessing they're not the issue). Any ideas?
Title: Re: Hoffman Blues Junior with No master Volume and No Meat
Post by: shooter on May 16, 2022, 04:39:20 pm
scope?


meter that can measure AC?


stick a 1khz signal in at the input jack, use your scope/meter to measure the Vac at the jack, adjust the signal to ~~100mVac-rms
turn volume/gain to 5, set all TS pots to 5
Move the scope/meter to the speaker and measure the Vac there and post results

Title: Re: Hoffman Blues Junior with No master Volume and No Meat
Post by: etliharr40 on May 16, 2022, 05:15:28 pm
Quote from: shooter on May 16, 2022, 04:39:20 pm
scope?


meter that can measure AC?



 No scope at the moment unfortunately. Meter does read AC. Any other troubleshooting methods without scope? I may be able to snag one from a friend. I’ll have to ask around.
Title: Re: Hoffman Blues Junior with No master Volume and No Meat
Post by: shooter on May 16, 2022, 05:22:17 pm
Quote
Any other troubleshooting methods


How to double check your amp wiring and find things you missed (el34world.com) (https://el34world.com/Forum/index.php?topic=17701.0)