P
pjd3
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Joined 2019
2021-05-09 12:18 am
#5
- 2021-05-09 12:18 am
- #5
HT
My initial confusion was in seeing HT of 340-0-340 and/or 360-0-360 in schematics of the Leak stereo 50 but continually seeing the claim in a few places that the plates of the EL34 are seeing only 380 on their plates. I've only built 2 guitar amps where I would make a decision on how much emission I wanted from my power tubes and thus choose a PT and rectification that would get me there. It took me a while to realize that the source of OT primary center-tap was taken from after the C-R-C filter where I was in the habit of assuming the voltage to the OT primary center tap to be taken at the first cap usually before a choke. Through my confusion I ended up answering my own question.
As far as intent and as far as I can tell at this point, I'm looking to build a stereo amp for a "fretless Ebow guitar" that I built for single note string like melodies, and wanted to first hear how this performed through hi fi type amp. I happened upon an old Leak stereo 50 and thought this would be good way to go forward with this learning experiment without having to spend hundreds of dollars up front for transformers. And I did intend on starting with ultra-linear and take it from there. So, although I have an intent, I do not know yet the means to the end. Since this instrument is very unique in its performance elements, I thought it may take some trial to land on a proper amplification set for it. And its a great opportunity to learn through experimenting instead of just following schematics and classic modifications.
Best, and thank you for coming by - and certainly, if there isn't enough juice with this Leak PT, I'll vye for another one
Phil Donovan
Phil Donovan
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pjd3
2021-05-09 1:17 am
#6
- 2021-05-09 1:17 am
- #6
starter circuit
As a starter I was going to set up a Mullard HF50/60 circuit. (or was that Eico?)
I just find the circuit interesting.
I have an EF86, a NOS 6SN7 and 4 Mullard EL34' so it would be interesting to hear how a channel of that would work. I'm also very interested in hearing the difference between these OT's in ultra linear verses pentode mode. Where this instrument is only one note at a time as opposed to a full audio bandwidth program I have no idea if I would even hear a difference, but, that's to be seen. Its said that there may be more output in pentode.
Tonight, or, right now actually, I'm trying to make sense out of the Leak stereo 50's power transformer. The primary is definately a different beast than anything I've worked with and sure don't want to put potential in the wrong place. Once that gets understood, the secondary coils seem fairly intuitive - just want to see that expected voltages are present.
Thanks for coming by - this part of amps is new to me, figuring out vintage components and circuits so, thinking out loud here, I appreciate your patience.
Best,
Phil Donovan
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pjd3
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Joined 2019
2021-05-09 3:32 am
#7
- 2021-05-09 3:32 am
- #7
They all work
so that was nice, mystery solved (probably wasn't even a mystery)
The PT to the trashed up Leak stereo 50 has all working secondary coils. Yippie!
I was most wondering what I'd find for HT voltage since one schematic showed 340 and another showed 360. I found (no loads at all, all open secondaries)
HT = 363-0-364 vac
preamp/power tube heater - 6.7 vac
Rectifier heater - 5.3 vac
Should the HT decrease by the same percentage as the heaters would decrease reaching their ratings, then the HT would actually be closer to 340 (343 is what I got using the ratio mulitplyer which was almost the same for the 2 heaters).
Not certain if it works just like that but, don't see why not.
Well, final thought is that this set of PT and 2 UL OT's look quite good to use in this indeterminate project coming up - to start with a hi-fi circuit like the Eico HF50 or Mullard 520, and see what various mods, additions, subtractions to the circuit yield in making a nice amp for this odd instrument I made.
Going forward I will find your input very valuable especially foreseeing any trouble I could get myself into. Going to try like hell not to fry anything that's costly to replace on the way. Wish me luck, I'm sure going to need it.
Thanks for reading
Best,
Phil Donovan
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PRR
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Joined 2003
Paid Member
2021-05-09 5:31 am
#8
- 2021-05-09 5:31 am
- #8
Frankly I suspect the *speakers*, and the preamp, make more difference than the power amp, right up into the edge of clipping.
At least that was my opinion PA-ing string ensembles mild and wild.
If you are going to over-over-drive like Neil Young in a hurricane, then power amp details matter.... and then your fretless ebow may be a moot frill when it is ALL LOUD.
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pjd3
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Joined 2019
2021-05-09 4:10 pm
#9
- 2021-05-09 4:10 pm
- #9
stereo amp
Hi PRR,
This system will only be used in a clean, low to mid volume capacity in stereo with stereo effects, mostly high quality reverbs.
The front preamp end will most likely be something like my Seventh Circle
DI - Neve (N72) - compressor - Lexicon or Kurzweil reverb/ambience processor into the amp. Thats just for straight tone with ambience from the instrument. I would expect some inclusion of impulse responce devices as time goes by, things like the Signal Wizard Systems Vsound to emulate acoustic wooden violin and cello bodies. For speakers, not sure at all how that will go. Could be anything to a 10" guitar speaker, to a 10 inch full range speaker, speaker and horn in a PA type cabinet, paper vs hemp, etc... That's where some trial and error will come into play I"m sure.
Its hard to say at this point just what percentage this system is to be a sound reproduction or, a tone producer. Its entirely possible that I will be adding in various controls to vary negative feedback, UL and pentode, a low pass filter (the instrument can generate alot of energy in the 9K region at full volume.
Also, I intend to build this up on a 19" rack chassis so It can be in proximity with all my preamps and processors. And being mobile is part of the idea.
These old tube amps I aquired were from a fairly well known EE and recordist named Scott Kent. He was known as one of the "Revox 77" guys. He came up with some upgrades that increased the headroom, lowered distortion and such, and people from all over would send their Revox's to him for the upgrades. His house was a hoarding extravaganza. I've never seen so much old electronic stuff in one house. Scott had every intention of refurbishing all of these amps and devices that he had aquired but became ill some years ago and those things became difficult. Scott was also one of the guys you called to record the Pipe organ. He just knew where to place what mic where to achieve great recordings. And when he dies, his old associated from the audio engineering society got in there fast to help his wife get what she deseved for all those high end and vintage Neumans and other coveted items. I ended up getting some very old tube signal generators, capacitor and resistor boxes and things. Everything smells like the 50's. I wonder if Scott would be pissed at me for not restoring the Leak 50 or, is he proud of me for usiing the good parts for something a little "new". His wife Marcia is good with it all!
Thanks for coming by PRR.
Best,
Phil Donovan
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