Sunn Model T inspired Eico HF-20 conversion, nfb loop question.

[Foxfire]

so i gutted the Eico's front end all the way back to the power tube grids and rebuilt it with with the Model T front end. i wired it up as though i were using the "Both" input jack so both halves of V1 are in parallel and they go to a dual gang pot. other than that it's pretty stock up the the power tube grids. i tried using the Model T's NFB/presence circuit but i get bad low pitched motor boating as soon as i switch it on. on the 2 other amps i've built i got a high pitched squeal letting me know that i need to switch the primaries. my guess and hopes are that i just need to switch the 4 primaries but i'm unsure about the significance of the pitch oscillation. in other words, can a low pitched oscillation indicate needing to switch the primaries? or does a low pitched motor boat oscillation generally mean something else. i understand there a lot of variables so i'm happy to answer any questions i can...Thanks.

Eico_HF-20 SAMS.pdf

[loudthud]

The blue wire on the Eico transformer corresponds to the blue/white wire on the Model T transformer phase wise. There is probably too much feedback. Change the 22K resistor, R20 on the Model T schematic, to something higher (maybe 47K or 100K) until the oscillation stops or connect the feedback to a lower impedance tap.

[Foxfire]

thanks for the quick reply. i should've added that i had tried 100K in place of the 22K without any perceived change. i was looking at the reissue model t schematic and think i'll try those values along with cleaning up my lead dressing and what not. i kind of wired it up as an after thought because i historically don't have much use for presence controls. i generally like a softer distortion but wanted to include it at at least until i got a chance to hear it's affect on the sound.

i will say the excluding the trouble with the NFB, i'm pretty darn happy with how it sounds. i have always wanted a Model T (and will probably never be able to get one) so i can't say how this little guy compares with any real accuracy but i can say it sounds great with a Rat in front of it. i love the sound of a Rat in front of a Sunn so my 2 goals were to build this amp as close to the Model T specs as i could and to have it sound as big and beefy as what i know a Rat driven Sunn to be. it's already sounding better than i was expecting and i really haven't even begun to clean and tweak yet.

[loudthud]

If the amp still oscillates when the feedback is disconnected, it doesn't have anything to do with the feedback in the output stage. Could be a ground issue. Pulling the tubes one at a time should give clues as to where the oscillation is. Do any of the pots affect the oscillation?

[Foxfire]

sorry if my reply was poorly worded, the amp sounds fine with out the feedback. it actually sounds really good. i'm hoping to get a bit of feedback to soften the gain/distortion a touch.

however, unrelated to the feedback i was just playing the amp and decided to crank it just to see how loud it can get. almost as soon as i finished strumming a chord the sound cut out so i shut the amp down. i turned it back on and i think i strummed another chord and i got a spark/arc in one of the power tubes. i shut the amp down again and replaced the tube just to make sure nothing outside of the tube was affected. i didn't try it maxed but at a reasonable volume everything sounded fine. i didn't replace any of the power section components but i think i'll have to take a closer look.

Edit- just tried the amp again at full volume and no problems. I'm still gonna replace the original components though.

[Foxfire]

one more quick reply before i head off for tour, i haven't had anytime to work on it and i am getting a bit more hum since i replaced the one power tube. i'm thinking that's do to the tubes not necessarily be matched, they're pulls. aside from that with a rat pedal in front of it it sounds absolutely incredible to my ears. i can't wait to get back from tour and finish this thing up.

[loudthud]

Ultra-linear outut stages tend to not reject hum as well as normal push pull output stages even of the tubes are matched. That's why many times you'll see a Pi filter network (cap-choke-cap) between the rectifier and the output transformer center tap. The choke in a 60W Sunn amp is 1.5Hy IIRC. In the Model T and the 120W amps, they used two in parallel.

[Foxfire]

Thanks for that. I grabbed another pair of 6l6s that I felt were better matched and they were a touch better but not much. Then I went at it with my chop stick and moved a few wires whiched helped a fair amount. The hum is now more than live able. I have plenty of room so I could always add a choke someday down the road.

I'm amazed by how much low end this amp has. Even on my orange or120 I generally max the bass and keep the treble around 9 o'clock but with this amp I get a ton of bass almost immediately. Which is fine with me...so far this amp has gotten me closer to the "sound in my head" than any of the many amps I've had over the last 10 or so years.

[Foxfire]

i'm back and i ended up replacing the 22K resistor with a 220K resistor. i wired up a 1M pot to find out how low i could go before it would become unstable and 180K was about it. i haven't gotten an real playing time on it yet but it does seem to have softened the amps natural distortion enough for my taste. i have the output jacks and presence pot in temporary mountings so i should be able to clean up the wiring once i get them mounted permanently.

[Foxfire]

ok so i more or less buttoned this amp up this morning and i'm very happy with it. over all it does exactly what i want it to do. however i have a few things i want to double check.

first, as i said earlier i have a bit of hum. it's so low that it's almost a non issue but i feel like i probably can get rid of some more so why not though i should say that i do not have a scope so it may be out of my ability. when i turn the amp on (no standby) it's dead silent for 10-15 seconds then the hum swells and fades then settles to a constant faint low hum, like a low organ note and the hum gets louder as i turn up the volume and the bass pots.
1. the hum is completely gone if i pull the power tubes.
2. if i pull V2 tube the volume of the hum is there but no longer affected by the volume and bass controls.
3. i've tried different tubes in all the positions accept for the rectifier.
4. i have an elevated artificial center tap on the heaters.
5. V1-V3 are on a ground bus grounded near the isolated input jack.
6. V4 and 5 are on a bus with the isolated output jacks and is grounded to the chassis at the filter cap cans ground.
I remember reading a few really helpful posts about diagnosing hum but I did not bookmark them and after looking at 7-8 pages I still haven't found them...I started writing this prior to isolating the bulk of the hum to V2 so I'll be checking everything around V2 that I can think of tomorrow.

Second, as Loudthud pointed out Sunn used a 1.5Hy filter choke in their 60watt amps. The closest one I've found that will work with my B+ is the Hammond P-T159T, 2.5Hy, 500VDC and 300ma. I understand what chokes do and more or less how they do it. What I'm not sure is if they are like filter caps in that you need one that is big enough to do the job but not so big it makes the amp too stiff?

Third, the Model T has a voltage divider in front of the line out jack in the NFB loop. I am not putting a line out jack in my amp but because the voltage divider is in the circuit regardless of the line out jack I decided to put a 33k resistor to ground figuring it was close enough. Any reasons for doing it differently?

[Foxfire]

i checked for leaky caps and everything looked good. i'll try to get a good pic or 2 posted today or tomorrow...