A year or two ago I converted my SE express-ish amp into a P-P with 6v6's. I'd ordered some polystyrene caps (Edit: meant to say polyester) and today I finally got around to putting them in. I wanted to try some larger values as the amp was still overly bright. Originally I had .022's throughout. I put a .1 in the PI a while back and that seemed to help. So today I replaced all of them with the polystyrene's. .022's in the EQ, .0022 in the second gain stage and .1's in the PI and output. Was definitely a step in the right direction. I had an extra .022 so after playing for a bit, I put that in place of the .0022. Made a huge difference, maybe too much. I may have to try a .0047 or something.
In any case after making that last swap, after the first few chords the volume dropped dramatically and I was getting this crazy chaotic oscillation. First thought was that I'd moved a wire or the extra bandwidth of the caps was causing some instability (it is an express after all) but after probing around I found that the OT primaries had shorted. Yanked it out and when I tested it again it read ok. Fiddling with the wires more made the short appear again. Its a Hammond 125E btw. I'm guessing the winding's got hot and the lacquer melted off near the solder lugs causing an intermittent short. Kinda sucks but at least its only a $45 transformer. Tubes appear to have survived. Curious as to why it failed? It's a 15w transformer rated for 70ma idle bias current. Tubes were biased at 28ma each. Plate voltage is around 385v. I'm guessing it has to do with the extra bass response from the bigger caps, but that may have just been a coincidence. Its possible I misread the impedance chart and used the wrong secondary connections. Will have to double check that tomorrow. Should have been setup for around 8k or so, gonna smack myself if it turns out to be incorrect.
Upside is I was thinking about trying a different transformer anyways. I've used the 125E in a few different builds and they seem to sound okay, but I was never really floored by the amps I built around them. So I'd like a few recommendations for an OT that would match well with a pair of 6v6's and the express circuit. Something around 20w. 4,8,16 ohm taps would be nice, but not necessary. Was thinking the classic tone tweed deluxe or deluxe reverb transformers might be a nice choice but not sure. Curious to hear your recommendations.
Last edited by strelok on Tue Apr 17, 2018 7:10 am, edited 1 time in total.
Their transformers typically get a pretty good rap. Though you might have been pushing quite a bit more than 15 watts through it, the clipped signal would probably exceed 15 watts.
Years ago, I built something similar using a Princeton output transformer but the transformer lacked bass response, perhaps Allen's amps might have a transformer that you are looking for?
The other thing is did you wire it up correctly? There seems to be quite a few options with these transformers. Below is the range of primary and secondary impedances.
Primary impedances from 1,200 to 25,000 Ohms.
Secondary impedances from 1.5 to 15 Ohms.
Thanks, I doubt its a HF oscillation. I've had that amp on the scope many times and I've never seen any HF noise on the output. I suppose its possible when I changed the caps out that something got moved or whatever to cause it but I was pretty careful not to move any wires around. On the scope and a resistive load the amp only made about 13-14 watts, so it was probably limited by the core saturation of the OT.
I checked the impedance chart again and it did turn out that I had it wired for about double what I thought it should be (16k instead of 8k). Somehow I misread the chart. So I feel dumb for that, but I'd be surprised if that's what killed it. Could see it if it where the other way around. Anyways not gonna sweat it, its a fairly inexpensive part and now I'm excited to try something different. Seems like Allen's amps has a couple transformers that might be a good choice (thanks for that suggestion) so I'm currently debating between them and classic tone.
Reeltarded wrote: ↑Wed Apr 18, 2018 11:02 am
They prefer transgendered.
I was wondering if anyone was gonna pickup on that.
I'm thinking the beefed up Deluxe transformer (#TO26) from Allen Amps will probably be a nice choice. Its a little more expensive than the classic tone models and some of his others but I think the extra heft might be worthwhile.
If you want good Bass response on the low E string and in turn better dynamic range then get away from a low core mass 15 watt OT and into something in the 25 watt or better range!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Mark wrote: ↑Wed Apr 18, 2018 11:47 pm
We had the whole gay marriage thing in Australia late last year. I don't want hear anything gay for a long time. What a debacle!
Three restrooms
Men
Women
Debacle
This whole 'me' thing in society is truly a debacle. Here in America; cultural cannibalism. They snuck one by after the revolution and it's grown into a machine that eats itself. Transformers. For sure.
Huxley... haha.. what a card!
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
I suppose I should elaborate, I have nothing against anyone, gay people are like any other people, but the whole process was a debacle, in one of many instances, one woman sack another woman working for her because she posted on Facebook she was voting no. When I wrote my post I read the woman got off sacking her employee because of her beliefs. I still think this sort of thing is wrong, people are free to have their own religious beliefs.
If it had been an open and frank social discussion I would have been for it, but all saw was intolerance and bigotry. Many arguments put forward were weak and emotionally based.
Anyway let's leave it there as I don't think there is much to be gained from it.
Oh no now look what I've started lol. I feel the same way. I just want the screeching to stop.
Stevem wrote: ↑Thu Apr 19, 2018 11:43 am
If you want good Bass response on the low E string and in turn better dynamic range then get away from a low core mass 15 watt OT and into something in the 25 watt or better range!
Yeah that's partly why I wanted to a different transformer to begin with, before this one died. I emailed David Allen and he said the TO26 is good for about 30 watts so I'll probably give him a call tomorrow. Nothing like a bit of misfortune to motivate some change.
Well I got the new OT installed this week. Sounds quite nice, less harsh in the highs than the old one and a bit more bottom. Thing doesn't feel like its going to take your head off anymore. Might actually consider installing the bright switch now. Also want to try some NOS 6v6's as well as see if I can get rid of some of the 1k grid stoppers just to see if I can get it to remain stable without them.
So I feel dumb for that, but I'd be surprised if that's what killed it. Could see it if it where the other way around. Anyways not gonna sweat it, its a fairly inexpensive part and now I'm excited to try something different. Seems like Allen's amps has a couple transformers that might be a good choice (thanks for that suggestion) so I'm currently debating between them and classic tone.
