....and Now for Something Completely Different!

[TUBEDUDE]

While in ghetto mode, how about a gap via hacksaw.

[JazzGuitarGimp]

While in ghetto mode, how about a gap via hacksaw.

[renshen1957]

While in ghetto mode, how about a gap via hacksaw.

Hi

It's been done, but this isn't a hifi amp.

Best regards

Steve

[David Root]

Sure, and I keep a razor in my shoe!

Finally got the AC cord and FSW on it today and cranked her up. Has some nasty PS buzz/hum, but sounds great!

The buzz/hum is odd in that if I touch V3 (driver) or V2 (12DW7) B+ nodes with the red probe on my DMM, with or without the black probe to ground, the hum/buzz disappears completely. Also the preamp caps common ground scratches in the speakers when I run the probe along the ground buss and there seems to be some interaction with the trim and gain pots.

I set the OT up so with an 8 ohm load it would see 3K. Then forgot about that and plugged it into a 4 ohm load, so the OT is seeing 1K5, which is a bit low (!). I should put a switch on the back panel.

Two 8 ohm 2x12s in parallel, each with a pair of old Celestion Blackbacks. Wind up the gain and the master to about 2 o'clock, awesome! I cannot believe a 10W amp sounds so loud and delightfully crunchy. But turn the master up and the gain down a bit and it cleans up nicely.

The KT66 draws about 85mA with plate at 352V, screen about the same, and cathode at 25.5V, so about 92% of Pd.

Driver screen is higher than expected at 80V, plate is 130V. This 5879 is set up for max. voltage out, not max. gain.

12DW7 B+ is 225V & V2B cathode is 129V, 3.3mA thru the 39K resistor to ground. Cathode 2.3V, 2.1mA.

V1 is about 10V low so I'll change the dropping resistor from 12K to 5K1 to bring things in line.

Heaters were high at 7.1V so I put a 5W 0.36 ohm sandbox in the line, now 6.4V, OK.

Miswired the FSW, I have yet to yet one right the first time! Getting 4.99V on the relay feed, from the 5V PT winding thru Colossal's excellent 5V relay PS board and relay board.

I'll have to work on the PS hum/buzz. Any suggestions/comments welcome.

[matt h]

(deleted)

[David Root]

Thanx Matt. I resoldered a few spots in the preamp caps ground line, no change in hum level.

Then I disconnected the relay from its board and hum/buzz is all gone!
I think I wired the relay contacts right, common to treble pot and top of V2A cathode resistor, NC not connected, 1uF cap and 390 pF cap connected NO.

Will try replacing the relay. Still have to get the footswitch working right.

I have also removed the choke I had behind the screen gridstopper because it
interfered with the UL, so now I have the UL working.

[Colossal]

David,

PM sent.

[Tone Lover]

I had some noise issues like you, that were from my relay.
Geezer recommended 100ohm resister off my neg. side of relay wiring no more noise. I actually tried it off both sides worked to quite that noice with no ill effects.
Bill

[David Root]

I have 150 ohm in the relay board. It looks like the FSW LED is either fried or wired backwards, (or both!).

EDIT: The LED was wired backwards.

[David Root]

This one has come a long way since two weeks ago.

Finally got the hummm/buzzz issue fixed this morning.

I had the preamp cap grounds tied to the preamp grounding buss bar over the pots, but all the other preamp grounds were connected directly to the preamp chassis ground point near the input jack. This must have caused a ground loop, although the buss bar is also connected at the other end to the same chassis ground point, so I don't quite understand how that works.

When I took the preamp caps ground off the buss bar and put it direct to a separate chassis ground point nearer the power end (Dumble style), presto change-o, no humm/buzz. LESSON LEARNED!

I had suspected the relay circuit but that turned out to be a red herring. However I also took the relay/FSW ground out to another separate chassis ground in the PS area just to be safe.

One thing that does not work is the 390pF cap to ground on the relay, the amp hums when it's connected to ground. So I did away with it. The 1uF cap boost on V2A works fine, but not like a PAB in the TS would. Only ~6dB boost. Surprisingly it does not pop when switched.

The amp sounds very good clean and sustains really well, a bit like a Dumble, when overdriven. Despite the UL it overdrives nicely but doesn't need any GNFB from the OT secondary. A worthwhile build, and especially as it didn't cost me hardly anything out of pocket that wasn't in stock. As someone wittily remarked, a "Beaks & Feet" build!

I've been advised to remove the 5K1 screen gridstopper on the KT66 as it interferes with the UL. We'll see if the amp sustains as well without it. Have a few other tweaks in mind, including taking the slope resistor down to 51K from 100K to bring the bass back a little.

Below is the current schematic and gutshot showing new grounding layout..

[matt h]

(deleted)

[David Root]

How about this progression? Just doodles on paper at this point.

[matt h]

(deleted)

[David Root]

Thanx matt! Blue Plates on the Hoffman forum had the same concern about the Marshall OT, especially in conjunction with 350VDC going thru it too..

Correct me if I'm wrong but two KT88s in that circuit might draw 220mA cranked right out, and any Marshall 100W OT designed for four PP EL34s should be able to handle at least 200mA. That OT weighs about 7 lb too. Come to think of it, some old 100W Marshall amps used 4x6550 for the US market. I'll admit these were all operating at 400-450V, not 350V so my current would be a bit higher.

I do have a 100W Edcor hifi 1.7K OT that weighs almost 10 lb but that is intended for a 4x6550 Steel String Singer one of these days. (100W -1dB 20 to 20kHz is approx equal to 150W at 40 to 10kHz).

Thanx for your comments on V3, the pentode/triode. By bootstrapping the CF do you mean with two 20K resistors instead of 100K? Might make sense as that triode is medium mu about 40-45. I could also use a 6CM8, the triode on it is mu=100, but the pinout is different than the 6AU8/6GH8.

[John_P_WI]

David, just remember your two KT88s can not pull more than the power supply can deliver. IIRC the 50 watt "marshall" OTs are rated around 160 ma / side. Could one not assume that IF your PT delivers 250 ma that the 100 watt OT would work in ghetto mode / parallel?