Recent Posts

81

Tube Amp Building - Tweaks - Repairs / Re: Bucking Transformer wiring

« Last post by jinchausti0 on October 19, 2025, 04:46:45 am »

Hi, is some time ago but wondering if you fixed this issue? i run over same problem with an old Yamaha JA1701 which as you give him power just stuck at higher speed. I suspect some diodes might be the issue. I have other units that work perfect with switching from slow to fast speed.

Any info will be appreciated thanks

82

Tube Amp Building - Tweaks - Repairs / Re: 6M5 6EM5 Valves in Push Pull

« Last post by TIMBO on October 19, 2025, 04:41:44 am »

Hey guys, Just when you think you are on the home stretch

 

83

Tube Amp Building - Tweaks - Repairs / Re: Planning single channel 6g4a

« Last post by bullkelp on October 19, 2025, 03:48:40 am »

Thanks
I think the bias tap is 52v
Why does fender use 2 x 8uf caps in the brown amp bias circuit?
Would I be ok just using my usual go to 50uf /150 v, And start with 470 ohm, a diode and a 10k bias pot / 27k tail and adjust from there?

84

Tube Amp Building - Tweaks - Repairs / Re: Weird distortion.

« Last post by tubeswell on October 19, 2025, 03:34:00 am »

400 on the plates
397 on the screens
-33 on both grids
Current draw 31mA and 33mA

13W is on the cool side for a 25W tube


What does a 1kHz output signal look like (on a scope)?

85

Tube Amp Building - Tweaks - Repairs / Re: Weird distortion.

« Last post by elams1894 on October 19, 2025, 12:14:52 am »

With weird farty distortion, the majority of the time I find the phase inverter either has a bung tube, or the PI plate resistors are toast. Especially if they are only 1/2 watt. I did have a big amp (130 watt Rockit) once that puked an OT. The distortion before it blew was weak and anaemic, with 2 of the 4 power tubes red-plating. Took the 15" speaker with it. It certainly was a different tone (more a prolonged death cry) than the woofy-farty-cheese type distortion of a bung PI.

It killed me to part with the 15" alnico JBL, so lessons were learned. Red plating 6L6's now haunt my dreams.

86

Tube Amp Building - Tweaks - Repairs / Re: 18W odd power problems

« Last post by HotBluePlates on October 18, 2025, 11:09:15 pm »

...  B+ voltages are up about 15% across the amp but the house voltage was almost identical, 122V yesterday and 121V last year.  That strikes me as a little weird, no?   

Main problem is the first stage is dead.  It has barely 90V at the plates of V1a and V1b.   ...

V1 is not "dead."  Plates have dropped from 128v to 90v, but cathode voltage is up from about 2v to 3.25v.  This indicates higher current through V1.
   2024:
   2.18v / 1.5kΩ = 1.45mA
   1.45mA x 100kΩ = 145v dropped
   Supply Node D:  275v - 145v = 130v ---> exactly what you have in your schematic

   Recent:
   3.25v / 1.5kΩ = 2.17mA
   2.17mA x 100kΩ = 217v dropped
   Supply Node D:  303v - 217v = 86v ---> exactly what is expected if V1 draws more current

Recent V2 measurements show higher supply voltage, and higher tube-current, matching the trend shown in V1.

Recent V3 measurements show lower tube-current (different 12AX7 in this socket?).

Overall supply voltages are up.  If not higher outlet voltage, then did you replace the 5Y3 rectifier?  A Sovtek type 5Y3 is actually lower-voltage-drop (it's not a "real 5Y3").  And if there was a change of filter caps, higher-µFs leads to higher-B+.


Nothing in your voltage survey points to a reason for "no sound."  In fact, everything looks like it should be working fine.  I'd look for an open coupling cap, or an open/broken wire from OT to speaker-jack.

87

Tube Amp Building - Tweaks - Repairs / Re: Planning single channel 6g4a

« Last post by tubeswell on October 18, 2025, 09:19:38 pm »

Depends on the bias winding voltage on your PT. As there is almost zilch current draw on a bias supply, you can count on a half-wave SS diode rectifier to rectify to 1.1412 x the secondary VAC at the reservoir cap. Whatever voltage divider you put in there after the reservoir cap will knock that down further. You'd be safe with a voltage divider output that gives a bias voltage range that's about +/-15V either side of 1/10th of the plate voltage of the 6L6s. So say your plate voltage is 450; that's a ball-park bias voltage mid-point of -45VDC with the range being -30 to -60. That's what I'd do.

88

Tube Amp Building - Tweaks - Repairs / Re: Planning single channel 6g4a

« Last post by bullkelp on October 18, 2025, 06:42:40 pm »

I think ive settled on running the full valve compliment...

What bias circuit would you choose for fixed adjustable.
I note the 6g4a schematic has a fixed 56k and two 8uF/150 along with a 15k.
Given im building from scratch and using a hammond 290DEX would I be ok just using the standard cirucit per this weber layout for the brown 6g13?
https://www.tedweber.com/diy-kits/

89

Other Topics / Re: Meme of the day

« Last post by shooter on October 18, 2025, 05:37:59 pm »

...

90

Tube Amp Building - Tweaks - Repairs / Re: Planning single channel 6g4a

« Last post by tubeswell on October 17, 2025, 07:05:12 pm »

I havent discounted that, im just not overly confident altering the schematic to install the FET.
Im ok with drilling. I just installed the IEC socekt easy enough and I have step drills and a press. Im just a little uncertain about the best spot for it

A enhancement mode mosfet like an IRF820 is pretty much a 'drop-in' replacement for a triode - especially a CF or a cathodyne. You just take the triode out and put the mosfet in. Drain = Plate, Gate = Grid, Source = Cathode. Bonus = no heater required, no H-K insulation to worry about. An IRF820 in a  TO220 package without a heat sink is good for 500V and about 1A. (And a 12V zener between gate and source is additional protection against over-voltage)