Recent Posts

41

Tube Amp Building - Tweaks - Repairs / Re: Any tips for taming a wild 5E3?

« Last post by isaac_teller on June 09, 2026, 06:16:54 am »

If you have a 12AX7 in V1 try a 12AY7 instead. If still too much' try a 12AU7.

42

Tube Amp Building - Tweaks - Repairs / Re: First amp build

« Last post by Mongrel714 on June 09, 2026, 04:39:35 am »

I'm not crazy about the bias power supply switching off with the Standby switch.

Yeah, and the fusing on the Secondary HT can be better, have a look at some standby and fusing ideas here:-

https://www.valvewizard.co.uk/standby.html

https://www.valvewizard.co.uk/fuses.html

Will this work? if not which way should I go??

43

Tube Amp Building - Tweaks - Repairs / Re: First amp build

« Last post by Mongrel714 on June 09, 2026, 04:36:23 am »

Neither layout in this thread gets the standby switch correct to the schematic they're noted to be based on.
Both disable the bias supply whilst in standby mode, thereby causing an enormous cathode current surge when standby is flipped.
Because the bias supply takes much longer to charge up than the anode and screen grid HT supply nodes.
Ok, got it!

44

Tube Amp Building - Tweaks - Repairs / Re: silvertone 1484

« Last post by johnfromcyrene on June 09, 2026, 02:14:54 am »

I posted the mojotone schematic and layout at the top.

45

Tube Amp Building - Tweaks - Repairs / Re: Bias Help for Rob Robinette JCM800 6V6

« Last post by sluckey on June 08, 2026, 11:35:33 pm »

My power transformer does not have a 6.3V Center Tap
The PT comes with standard 2 GN 6.3V Heater wires. Why do I need the GN/YE wire?

You need the center tap to reduce filament hum. So, make an artificial center tap using two 100Ω 1/2 watt resistors. Put the resistors on V5 socket. Solder one end of one resistor to pin 2. Solder one end of the other resistor to pin 7. Let the two resistors stand up and twist the two dangling ends together. Now solder a green/yellow wire to the two resistors. Secure this so the two resistors will not flop around and touch anything else. Now connect the other end of the green/yellow wire to the board as shown in Rob's drawing.

NOTE*** The wire does not have to be green/yellow.

46

Tube Amp Building - Tweaks - Repairs / Any tips for taming a wild 5E3?

« Last post by slim66 on June 08, 2026, 10:50:26 pm »

So, I built my 1st 5E3.  I used P-TGP-002 PT and a Hammond 1750-H OT.  The 1st filter cap is 33uf, and the power tube cathode resistor is a 10w, along with a 50v power tube bypass cap.  The rest of the circuit is standard.
     Going into a Celestion G12M 65 watt speaker, this thing is too loud with the volume at 1.  Also, like most 5E3's, it starts grinding at @2, and just keeps getting more grindy after that.

     What can I do to tame it a bit?

Thanks, 

Slim

47

Tube Amp Building - Tweaks - Repairs / Re: Bias Help for Rob Robinette JCM800 6V6

« Last post by rud85ny on June 08, 2026, 10:48:09 pm »

Back to this same circuit. My power transformer does not have a 6.3V Center Tap (GN/YE) wire as described in Robs layout. Everything else matches.
The PT comes with standard 2 GN 6.3V Heater wires. Why do I need the GN/YE wire? I will be using the solid-state rectifier setup. can this be omitted?
should I tap in a 6.3V connection from the regular filament heater/ lamp circuit?

48

Tube Amp Building - Tweaks - Repairs / Re: Bias Help for Rob Robinette JCM800 6V6

« Last post by rud85ny on June 08, 2026, 10:42:22 pm »

Copy that sir. Thanks a million!

49

Tube Amp Building - Tweaks - Repairs / Re: silvertone 1484

« Last post by SoundmasterG on June 08, 2026, 09:41:33 pm »

I think the mojotone uses diodes instead of a rectifier. what about adding a gz34 to create sag for the amp?

The stock Silvertone 1484/1485 amp with the multiple voltage doubler stages creates sag all by itself, even with the silicon diodes. A voltage doubler doesn't have much sag until the current demands get high, like when you have the amp way up loud. When the demands get too high, the doubler circuit will sag even more than many tube rectifiers. In the 1484 and 1485, they used two doublers stacked on top of each other, so you have even more sag than usual when the current demands go up. Add to that a really small output transformer that doesn't have much low or high frequencies in comparison to a larger transformer, and the fact they are using a 6CG7 for the phase inverter in a cathodyne circuit, and preamps with lowish gain, you end up with a unique amp. The fact that the screen voltage is using only a couple of the doubler stages instead of all of them limits the power and makes the tone a bit muted and soft. Moving this to the same tap as the B+ like a Fender does ups the power, but not much. These amps are really only about 30 watts RMS in stock form. Moving the screen tap gets you to about 35 watts, maybe, but it increases the risk to blowing the stock output transformer. That said, an amp I modified for my brother years ago does this along with many other mods, and it is still working on the original transformer.


I'd guess the Mojotone circuit was likely using different phase inverter tubes than the 6CG7 since no one makes it these days. You could sort of duplicate it with a 12AU7 but it doesn't have as much current capability. I couldn't find the kit on Mojotone's site, and their link for their schematics went nowhere also.


Greg

50

AmpTools/Tech Tips / Re: Heathkit T3 signal tracer

« Last post by goldstache on June 08, 2026, 07:32:31 pm »

Thank you so very much for the thoughtful posts!


Here’s where I landed.


So turns out that my shielded cable type mattered.
Went to an RG 70R type, and optical deflection of magic eye was same regardless of AC mains phase input.


Why? Could not say……..


Once the irregularity seemed handled. Wired modern convention
Live/fuse 750ma slow (as I have mains current shunt rig, and won’t be using the heathkit for mains draw)/switch/transformer.
Neut to transformer.


Cap removed, though was good on Sencore.


I did try the no cap first. To see if there was a leakage current. Just to parse what the heck.


Then continued on to the probe discovery.


I’m just using it AF mode. Noise test.
I do have an RF probe, and verified.


I did cap isolate the AF tube from gain pot wiper, had a 10M handy for bias.
And the other oddity has I worked it, was that the unit had a 6x5 present for recto.


In my clamoring for info, noticed heathkit had 2 schemos. Prolly more.
But the other schematic had a 6y5.
And the primarily 12v heated tubes needing the full filament winding, end to end.
The CT was left floating for the 6x5 to one end, to warm.


The 6y5 scheme has that CT at chassis.


Despite having the 6x5 version wired and in place, I decided to 0v that filament CT.


So recto is running
6.3vac - 0v


Instead of the 6.3 -0- ………(6.3v)
Gotta help with some idle noise in unit???


And, all that….
I like it.


It’s really good at finding trash passives (new band name maybe)


Thanks to the both of you. I will carefully read what ya sent a couple times, was just exuberant to share that I got to put to bed…for now.