Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Grantorino on November 29, 2025, 03:28:32 am
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Hello EL34 World.
An original Fender Blues Deville 4x10 combo is exhibiting very low output and some switching issues:
- On the clean channel, the Volume knob works and the master does not
- On the Drive channel, the Master knob works and the Volume does not
- On the Drive channel, the drive knob affects distortion as expected
- The reverb works, as do the EQ controls.
State on arrival:
- No output. Speakers wired incorrectly. Voice coils in two speakers are open/shorted
- Bulging capacitors, broken input jacks, broken control pots (spinning 360 degrees)
- Two 5W dropper resistors had visibly overheated and charred the PCB beneath
Work done:
- Disassembled amp, labelled flying leads for reinstallation later
- Replaced all electrolytics, bias supply rebuilt with tubes biased correctly. B+ supply voltages healthy and stable
- Two 5W dropper resistors replaced
- Cab temporarily wired to run on two speaker configuration
After reassembly, output was restored to the amplifier but the low output and switching issue made itself known. I disassembled again to begin the work of following the schematic and testing connections, highlighting the connections confirmed.
Additional notes:
- All connections on the 3 ribbon cables have continuity between main board and tube board,
- Each of the pots test correctly, including the volume pots
- I have inspected for cracked joints and lifted pads, reflowing many connections
Due to the nature of the issue, I suspected the relay switches RY1A and RY2A may be at fault, or perhaps something in this switching circuit. Today, the plan was to test these components and keep following the fault-finding procedure of testing connections and highlighting on the schematic. I started from the volume pot.
I'd appreciate any advice on where to focus signal-tracing and whether a faulty relay or switching circuit could cause the behaviour of low output + Volume/Master pot strangeness.
Thanks again.
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Look at how the relays are configured and you will see that the your volume, master and gain controls are working correctly.
As to your low output, you did not mention whether or not you tested the tubes or are using new tubes or what. Also, are you connecting to a resistive load and examining an output signal with an O-Scope? Are you using a DMM across the speakers to measure the signal? How are you defining "low output"? Simply listening to volume out of the questionable speakers?
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Thank you for your reply Brian and for reframing the focus away from the switching to the low output. Back to fundamentals.
To test the speakers, I used a DMM resistance measurement:
• Speakers identified as good measured 6.3 Ω
• ‘Dead’ speakers measured 0 Ω
• Using the 9V battery test, the good speakers showed a strong reaction while the dead speakers showed no reaction.
I therefore wired the two good speakers in parallel.
As a different load, I could run the amplifier into a Celestion 16 Ω speaker I have on hand.
As for the tubes, I do not currently have a spare matched pair on hand.
I did switch around the pre-amp tube positions and saw no improvement.
Then, I tested each of the ecc38’s from the Blues Deville by placing them into V1 of my Marshall DSL40. They each ran well. I don’t yet have a tube tester.
When I say low output, yes I mean low volume.
As the chassis is currently disassembled with all flying leads disconnected, I had planned to follow the schematic to identify the problem area, whether it be the switching, power section, or signal path.
From your response, it seems the switching is not the issue and I thank you for helping to rule that out.
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I'm not sure if you are using the term "fly lead" correctly, but to properly test the output/functionality of the amp, you should be connecting everything as it would be when the amp is being used.
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Sure Brian - I haven’t been schooled on correct lingo, so i’m sorry if my post wasn’t clear.
By flying leads I just meant the leads that aren’t soldered in place, they clip in. From the transformers etc. There are many to the board, now unclipped and labelled.
Of course i’m aware that the amp needs to be reassembled for testing. I had done this once previously following the restoration work, when I discovered the low output issue.
I suppose i’m delaying reassembly again as nothing has been done to ameliorate the issue and it is quite an involved process of disassembly/reassembly.
To me, the issue isn’t indicative of failing power tubes.
Hoping someone can chime in and point me in the right direction!