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61

Tube Amp Building - Tweaks - Repairs / Re: Reverb bleed at 0 on RVB pot on a '65 blackpanel Fender Vibrolux

« Last post by sluckey on March 09, 2026, 01:49:15 pm »

This sounds like a non-issue to me. Teach the customer how to use the footswitch.

Connect a gator clip lead between chassis and the wiper of the reverb pot. Does this kill the reverb?

Also replace the 25µF cap that connects to pins 3 and 8 of V4.

62

Tube Amp Building - Tweaks - Repairs / Re: Why did Marshall use the EL34 vs. the 6L6?

« Last post by pdf64 on March 09, 2026, 01:13:13 pm »

I suspect Brimar never made a 6L6GC (the 5 layer anode technology seems like it may be unfeasibly expensive to implement at low production volumes https://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2015%20No%201.pdf).

63

Tube Amp Building - Tweaks - Repairs / Reverb bleed at 0 on RVB pot on a '65 blackpanel Fender Vibrolux

« Last post by Sansteeth on March 09, 2026, 12:19:40 pm »

Hello everyone,
I'm working on this beautiful blackpanel Fender Vibrolux (unfortunately all transformers have been changed and while no "mods"were made, lots of parts are not original inside).
It came in because the customer was complaining he could still hear the reverb when the reverb pot was turned all the way down.
However, I found out the original reverb pot was only going down to 700ohms and the 220k CC resistor to ground coming after that was measuring 272k (RVB recovery stage grid leak resistor).
I changed them both, now I get a solid connection from the wiper to ground when it's turned all the way down and yet I am still getting reverb bleed. It's subtle, but it's there. The footswitch gets rid of the bleed but I don't understand why the pot cannot put the reverb signal to ground all the same and get the same result.
If I lift one leg of the 500pF at the input of the reverb circuit to feed it a signal and look for it on the other side (without the dry signal), I need to crank up the resolution and my scope (analog) gets too hashy on the other side to find where the bleed happens on the circuit unfortunately.
I've had cases of conductive boards on Fender amps lately but mostly silverpanels, but now I'm paranoid the reverb pot works fine, but somehow the grid of the recovery stage picks up the wet signal somewhere on the board.
Have you folks had experience with Fender blackpanels where you just can't fully dial off the little bit of reverb tail left without the footswitch?
Thanks a lot for your input!

64

Tube Amp Building - Tweaks - Repairs / Re: Why did Marshall use the EL34 vs. the 6L6?

« Last post by Merlin on March 09, 2026, 09:28:41 am »

In the 1950s British appliance manufacturers were slapped with export quotas by the government. Grampian were required by law to sell only a fraction of their products to British consumers, the rest had to go abroad.* A lot of mass-produced products were therefore aimed at the American market, and I suspect that is where most Brimar valves went. Marshall weren't in the same league as radio/TV producers of course.
*Yes, this was as economically dumb as it sounds.

65

Tube Amp Building - Tweaks - Repairs / Re: Carlsboro tube reverb noise/Hiss

« Last post by Jennings on March 09, 2026, 06:12:00 am »

Start by checking all the resistor values and so on...they'll be carbon composite, so likely the values will have drifted high.  Test your valves too.  Replace any resistors that have drifted more than 10%.  I'd also replace all the electrolytic capacitors.  I'd probably also replace the plate resistors with metal film types.  The shielded cables to the reverb pan, and pan itself, will need to be located as far away from the mains transformer and AC wire runs as you can get them.  However there is likely more hiss/hum than in a combo as there's only so much distance to play with in a head format.  Your schematic has handy voltage indications, so you'll be able to take test voltages and compare at various points.

66

Tube Amp Building - Tweaks - Repairs / Re: Why did Marshall use the EL34 vs. the 6L6?

« Last post by Jennings on March 09, 2026, 05:54:32 am »

In the 50s and early 60s there were a number of utility public address type amps, for use in factories or offices and so on, that made use of the 6V6 and 6L6 in the UK.  Grampian, for example used them in several models.  And they do crop up in radios (6V6 mostly through).  But largely for the reasons above they stuck with European valves.  Brimar in the UK did make/re-brand US style tubes...their name was a compound of " British Manufactured American Radio" to reflect their main aim of making US designs available to the UK market when set up by STC.

67

Tube Amp Building - Tweaks - Repairs / Revisiting Sears Silvertone 1484/85 Piezo Tanks.

« Last post by DummyLoad on March 09, 2026, 01:07:47 am »

From a thread from many years ago: https://el34world.com/Forum/index.php?topic=9951.msg93741#msg93741


Did my own take on the issue. My amp is a model 1484; mods should work on model 1485 as well.   


Moved the reverb signal source from the top of the ch2 volume to the wiper since, IMO, the reverb sounded a bit too thick at lower playing volume with mods.
Remove the paraphase divider network R47 & R48 & parallel the 6CG7/6FQ7 driver triodes; add a 10uF bypass cap to driver cathodes; replaced the pair of 27K plate Rs, R45 & R46 with single 15K 2W part; 3W/5W part is best there; 2W is what I had in stock and works for now.
Removed R40, 560K divider, replaced R41 68K with 220K (my amp had a 150K for R41).
I used a 8FB type tank; going to test with a type 4FB, I like the tone a bit better of the long tanks.
Plan to use a TRS 1/4" female jack to chassis and use a 1/4" male TRS to RCA jack splitter and mount the reverb tank in the speaker cab, right now tanks is connected with modified RCA patch cords with test clips.


Sounds really nice, much like the verb of my Ampeg VT40.

Link to schematic I used.

Link to schema of mods.


--Pete

68

Tube Amp Building - Tweaks - Repairs / Re: Why did Marshall use the EL34 vs. the 6L6?

« Last post by SoundmasterG on March 08, 2026, 10:25:35 pm »

The El34 is a 25 watt tube with a current draw of 1.5 amps

The 6L6 is a 30 watt tube with a current draw of .9 amps


I'd think you could go with the same PT to run cooler with the 6L6 and get more power


Why do you think they used the EL34 over the 6L6?

The choice of EL34 vs 6L6 was down to availability and price as the others have mentioned.  The EL34 can make more power than the 6L6 with a pair able to output up to 100W whereas a 6L6 pair is more like 60 watts. The plate dissipation numbers that you quoted don't correspond to output power per se.


Greg

69

Tube Amp Building - Tweaks - Repairs / Re: Why did Marshall use the EL34 vs. the 6L6?

« Last post by bmccowan on March 08, 2026, 08:47:45 pm »

So is it tubeswell or Tubeswell?

70

Tube Amp Building - Tweaks - Repairs / Carlsboro tube reverb noise/Hiss

« Last post by Yosser on March 08, 2026, 02:24:07 pm »

The reverb appears to be very noisy and rather tinny sounding, is this normal? I'm wondering if it's to do with the transistors