Loesch 300B amp ID?

Shot in the dark here - but I've read some people on here mentioning that they have experience with some Arthur Loesch 300B designs.

I've got an opportunity to buy a pair of amps, however the owner doesn't have the schematic or ID for the output transformer... I tried rooting around the web to see if I could find the design based on tubes/power transformer, but no luck yet. I thought I might post here in the (off?) chance someone might be able to shine a light on it, or sound out any ideas. I am aware giving a priori statements on an amp and its performance without any schematic is tricky to impossible. In any case, I might try to track the schematic down through the original owner/seller (from a hifi store/dealer, not DIY I believe).

Tubes are:
2 x 6J5GT
5AR4
300B

Power transformer is Hammond 274BX. No markings on Output transformer. Any pertinent questions I could ask the owner to get an idea of what it is? 8 ohms with another unused + unidentified tap. Total weight of amp is 40 lbs plus each.

I'll be able to hear the amps before any sale, but as a first time amp buyer I am a little wary. Especially curious and want to go down the no feedback route, but don't have the knowledge to confirm if this amp falls into that category.
 

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paulbottlehead

Active Member
I would expect that the circuit looks a lot like the JE Labs 300B, especially given the date of manufacture.
The output transformers may have been made by a company operating back then called "Handmade". They still owe me some money! :-/ They do not look like Hammond transformers, and the blue, black, and red secondary wires rule out so many manufacturers who would have otherwise used the black/green/yellow/orange standard coloring.

I wouldn't be very excited about shipping amps like that, as I would bet the chassis plates are not particularly thick but they are enormous and the iron is quite large. Since you are local, you should definitely give them a listen to see what you think, but I would not have expected the parts cost for that pair of amps when they were built to exceed $600.
 
Thanks for the further notes @paulbottlehead , that's useful. Joe Levy shot back an email saying he didn't recognise the design as one of Arthur's, but that it might be a one-off build etc., and that he might have notes in a file.

Offered price is $1600, but I'm having slight second thoughts - ideally wanted to go the snobbish Allen Bradley, mostly Paper in Oil caps, decent OPT, NFB etc. and actually confirmed design - preferably a WE91 variant. At least at first to get an idea of that route, and know if I'd be missing anything... And I'm only really looking for a mono amp (tiny apartment). You're right about the size! - It's 90lbs combined... 70 miles out of town.
 

marantzfan

Administrator
Staff member
I have to agree with @paulbottlehead on this one; I don't see enough there to justify the asking price. Decent but not great parts selection, unknown OPT's... If it were my money I would pass.
 

paulbottlehead

Active Member
The WE91 is not a particularly good circuit by modern standards.

$1600 isn't exactly a screaming deal. That would be a decent deal if we could look at them and see Magnequest, Tango, or Sowter output transformers, but otherwise it's very hard to really know what you'd be getting. I do also notice that the chassis is used as a signal path conductor in these amps.
 
Thanks, I'm leaning towards not taking it. Originally was $200 more..

I guess it's just a curiosity for my part on a WE91 inspired amp, from Sound Practices #1 etc. Regarding shortcomings from a modern perspective that you've found, does it mostly come down to roll-offs at either extreme of the frequency range? Slight veiling etc, bloated bass etc.? I could probably live with those, just looking for a lively and vivid amp to mate well with an Altec 755A and small 802/414/32 2 way to be honest. Midrange being the focus. And given it's a mono setup, I don't know how many concessions to modern practice I'm already throwing out the window haha.

But I'd be happy to consider other designs anyone could recommend in this vein, including the Bottlehead ones (I have a Crack with Speedball you recently repaired [or other Paul?], that I've been enjoying.)
It's tough not being a consummate DIY-er though - hard to know what to shoot towards without actually hearing the amps. I'd likely end up commissioning a build.
 
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paulbottlehead

Active Member
The rolloff up top is very pronounced and was intentional EQing for the intended design of the amp back in the day. They start rolling off around 7kHz. You can triode strap the driver tube and all of that goes away. The original softness of LF response in the 91A can be resolved by just using different iron.

(I am indeed the repair Paul)
 
Thanks for the comments. That's especially good to know re. strapping the driver tube. I'll bear that in mind!
 

paulbottlehead

Active Member
It does also require some circuit modifications, which I've posted on this forum elsewhere, but nothing beyond a couple of new resistors.
 

marantzfan

Administrator
Staff member
It does also require some circuit modifications, which I've posted on this forum elsewhere, but nothing beyond a couple of new resistors.

I believe that was in this thread:

 
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