Sag phenomena?

[Chuck H]

The JTM OT isn't a suitable substitute. Your OT is small, but it should be fine. I don't think a 300-0-300 PT running into a 5Y3 should be producing enough voltage to be "frying 6V6s on contact" if the amp is biased correctly. In fact, cathode bias should make it more forgiving of highish voltage unless the resistor value is just wrong for 6V6's to begin with.

[Howlin' Mad Mac]

It doesn't seem like your OT is too small unless you're after more watts, however (and please correct me if I'm wrong here) I think you could use a smaller PT, maybe something along the lines of a Tweed Deluxe tranny unless you just happen to have the other one sitting on a shelf and/or don't mind toting around the extra pounds.

[Danelectron]

Well, Is this phenomena a result of the rectifier tube being to small for the KT66s? Can I simply plug in a larger rectifier tube or are there other components that need to be adjusted to do this? I have thought about plugging in some 6V6s, but all I have are some NOS RCAs and don't really want to sacrifice them.

Sorry for the delay in responding. Had some family issues going on and haven't had time to get back to this.

[Chuck H]

A 5y3 is a little whimpy for a pair of big bottles. You could try a 5u4 for a similar voltage drop and higher mA capacity from the rectifier.

[Danelectron]

Chuck, I'll try that and post results. Thanks, Dan

[Bruce / Mission Amps]

Right.
Realistically, a NOS 5Y3GT is only a 125ma rectifier tube.
A pair of KT66s in class A, idling at around 60-80ma each, depending on the actual plate to cathode VDC.....should be able to max out the 5Y3GT at or slightly above their idle condition, leaving virtually no head room for a solid B+ node.
In other words... with respect to ground, measure your AC voltage to the rectifier tube, lugs 4 or 6 and then again at the output transformer primary leads to the actual KT66 tube sockets, lugs 3.
If those voltages are similar or the same, I'd use a better-stiffer rectifier like a 5U4GB or maybe even try a GZ34/5AR4.

Oops...It looks to be the case after rereading your post with some voltages..... 280vdc to the power tubes with a 300vac@250ma PT??? You have to get rid of the 5Y3GT.
That PT with a decent rectifier and filtering should have your B+ rail to the OT of around 365vdc-390vdc under this load.

[Danelectron]

Bruce, I ordered a 5u4gb this morning. I'm hoping I don't lose all the sag. I've been playing this amp for almost a year at living room volumns and it sounds great. It wasn't until I cranked it full blast that the wierd sag issue showed up. I guess it's a no brainer for most of you guys and I should have thought about it when my friend suggested throwing in the KTs. I've built a few small amps with 5y3s and 6V6s and I do understand it's OK to swap 6V6s in place of 6L6s, but not vice versa. This has been a good learning experience and if I lose too much of the present tone I will have the opportunity to mess with the voltage divider to try to get some of it back. One question, how will the higher voltages with the larger rectifier affect the output transformer? Thanks for your attention, Dan

[guitarmike2107]

I've been playing this amp for almost a year at living room volumns and it sounds great. It wasn't until I cranked it full blast that the wierd sag issue showed up.

Wow! that shows great discipline, my new amps don't last 5 minutes without getting cranked ... or until my wife goes out.

I doubt changing the rectifier will affect the living room feel of the amp, maybe a bit cleaner and bolder due to the added voltage, but you should notice the difference at louder voltages.

[tubeswell]

I love a bit of sag in rock amps especially on power chords. And a nice touch in a bit of Zep - e.g.: if you put it in black dog in any of the power chords at the end of the riffs, or in rock'n'roll, or the ocean or...

[Danelectron]

Wow! that shows great discipline

Actually, I'm a bit conservative in that I don't like to max the volumn past 9(roughly). Kinda like saving the last bullet for when you really need it. I've got a 100 watt marshall that more than covers my needs and several smaller builds. I just happened to be in the mood to see what top end would do so maxed it and discovered a parasitic squeal. I then added screen resistors which took care of that. I don't know why my friend didn't have them on the schematic and at the time I didn't understand the purpose of them. That was a good hands on learning experience. I ended up playing it at that volumn for an hour or so due to the weird pulsing pumping distortion. It really had a cool feel. The sustain was very controllable. I could lean in a bit and get it to ring then back off and it would fade out. It was just strange the way it would climb in volumn before the fade out. That made it somewhat limited for general use, but had fun doing a bit of a solo jam session. Though in the living room it probably drove the neighbors nuts. Between 9 and 10 there was quite a jump in volumn. On that note I really miss those whole album side jams like Humble Pie and The Allman Bros. and such. These new millenium bands could learn a bit by trying to write something longer than 3 minutes.

Tubeswell, I'm pickin' up what yer layin' down.

[Chuck H]

You can expect probably 20V more at idle and as much as 30V more when pushed using the 5u4gb instead of the 5y3. You'll want to at least check the bias with the new tube in. The amp will be a little louder, should be a little more dynamic at lower volumes and a lot more dynamic when pushed. A quick Duncan PSU2 simulation demonstrates the 5y3 crumbling under the current conditions. The 5u4gb will definitely sag, with the highest voltage drop of any common rectifier that can support your tube compliment, but it won't crumble.

[Danelectron]

ChuckH, That makes sense. I would have thought it would be a bit louder with KT66s than it is. I suppose the 5Y3 is just giving what it can. The 5Y3 is an old stock RCA. Maybe they had had a few more balls back then. Other than the extreme sag it's still hangin' with the big boys. I will definately recheck the bias. Thanks.

[Danelectron]

Update: I installed a 5U4 rectifier and the sag phenomena has been greatly reduced. The amp still has great sustain but the throbbing, climbing effect is gone. I ordered a 50 watt OT to replace the small 20 watter. Is there a way to introduce very excessive sag? Is this harmful to components or tubes? I could see a place for this effect on a limited basis.

[Chuck H]

You could try a big resistor in the power supply. Like 1K 50W on the CT of the HV wind with a switch across it to bypass the effect.

[trobbins]

Keep in mind the 'sag' sound character you observed could be due to guitar string resonance beats of some origin building up and dying down - possibly fed by speaker feedback.