It was the least expensive way to hit the "quality" amp mantra in 1950.
I am keener on BIG amps, as they can feed big hungry low efficiency speakers.
I think our Russian friend Anatoliy has a lot of it right:
http://www.wavebourn.com/why_pentodes.html
He especially holds the correct view, 6L6 oids sound no good until right on the limit of melting, which presumably is why Williamson gave up and connected them as low power triodes.
1940-50s Bogens are a lot rarer.
I mentioned this before, but I have this pair of HO125, which has been on hold, wondering how to tackle it, being as someone has already melted lots of bits in them.
807s had special anodes and a lot of other tweaks to make them stand up to intense abuse, which of course is exactly what I intend to give them!
Having ONE is already quite rare.
Having a pair is really a one off, and there are none in Europe.
(I have heard of one other person managed to acquire a pair in the USA.)
One was a rack mount, so it's missing the cage, and has been horribly bodged. Not sure what can be done to find another cage.
Here they are sitting on the floor of the store room:-
After careful checking and blowing fuses, it turned out, BOTH HT mains transformers had internal shorts.
This amp has seperate heater and HT transformers, but the heater transfos are OK, just need a small toroid to feed them 110V via a simple step down.
I have a surplus pair of 110V HO50 transformers, but they are simply not up to supplying so much current for a quad of 807s.
I have a toss up which way to rebuild them, but sure as anything now they will get a matched QUAD of 807s instead of the original heavily driven pairs.
Being as the original 807 bottle is huge, that inevitably means changing the sockets to brand new LOKTALs, adding an extra pair and drilling new holes with a purpose bought Q max punch.
The HO125 is the only one that is capable of being modified this way,- more space in the giant chassis!
I have already made up 2 matched quads of brand new NOS 5B254M (which actually cost almost nothing), each one being tested for current under working conditions in the old HO50 guitar mixer amp.
There were 2-3 which arced over inside from new, from a batch of 20+, which proves a point,- there's no point in buying sub standard Chinese or Russian modern stuff when even original industrial/mil spec UK made stuff has a high reject rate from new.
Here are shots of the incredible Bogen HO125 pair from underneath.
Beautiful tag board construction, making life easy for replacement of the various bits that have aged and died.
It's remarkably similar to the HO50, except for paralleled 6SN7 driver valves, a HUGE output transformer using the same but even larger cores used in the much later MO200, and of course anode HT of 835V on those top caps.
These are 70 years old remember.
Being as they are AB2, and suffer zero blocking distortion, so will give any 2020 KT150 amp from Upscale, Primaluna or Jadis such a serious run for its money, people will wonder what has happened in all that time.
They must have cost a lot in 1949, there's simply so much iron on there, which is why the amps weigh a ton!
Another funny thing.
People see the 90ohm, 70V and 140V taps on the back and give up!
People claim (of course wrongly) they can't be used for normal 8/16ohm loads, so they throw away an outstandingly good transformer.
Well, in those secondary windings they have 2 centre taps.
The com - 70v and 140V, of course the 70V is CT for 140V.
However, this is really smart, the 90 ohm winding (105V) is the CT for the 70-140V winding which gives guess what?
2 x 35V taps. ie 2 x 10 ohm windings for 125W.
Being as we now know the output transformer A-A load on the HO50 was an astonishly low 3.3K ohm, I measured the HO125 OPT, T254BU.
This came in at 5.2-5.3K.
I don't mind dropping the A-A load by connecting 8 ohm speakers on the 35V taps. That then gives a refected impedance of 4250ohms, - exactly half way between the Bogen MO100A and the quad 8417 M120,
(which also gives 120W,-I have a pair of those also).
In my opinion the 125W RMS figure is highly conservative because it only requires 800V RMS across the transformer.
Presumably, at which point the HT transformer ran out of current or the 807s themselves.
Being as the OPT are a full INCH bigger than those on the equally conservative rated MO200 (and the even smaller M120) - (with an octet of 8417s!) it's very clear the HO125 is the biggest beast, Bogen ever made, quite easily capable of close to 200W at right down to 20hz with likely very little saturation.
Here's one from a MO200 (same one as in MO100A).
With the top off?
Those come in at 4750 ohms A-A, which is why they make a good solid 110W RMS clean from 660V HT once the drive circuits are all sorted.
That is right on the limit of what the Octal sockets will stand, as witnessed by many arcs and burnt out anode resistors on one of the pair of my Bogen MO200s. (Yes I have a pair of those fantastic amps too!)
The Bogen HO125 looks very promising because not only do the anode wires come out of the top of the huge drop thru transformers but the sensitive bits are hidden well below the chassis.
Here is what it's slightly smaller cousin looks like with the same number of transformers (!):-
They really don't come up for sale very often.
It's a sort of "detuned HO125"
They have a slightly lower supply of 765V,(still over the 750V thought to be the max in ICAS service for 807s).
It explains the lower power output of 100W, useing the same paralleled 6SN7 driver valves, to supply all that grid current but no bias stabiliser as in the HO125.
That is the main problem with the MO200s I have.
There is such tiny space between the high gain AF amp gain stages under chasis and the output transformer wires, it takes nothing to set the thing off into violent and self destructive oscillation!
Been there done that!
So, the idea is to convert them to matched quads of 5B254M (smaller anode caps) and find a power transformer that is compact enough and powerful enough to feed them.
Finding a drop thru that can produce 820-900V stable at 400VA after a swinging choke is proving no easy task, because it has to produce 1000V AC with minimal derating at full load.
A typical transformer manufacturer gave me these derating figures:-
On the 100VA - 10%
On the 200VA - 9% = 910V - after choke input filter 820V
On the 300VA - 3% = 970V - after choke input filter 870V
In this case the target figure for the new amp is 175W RMS clean at 1% THD.
Here are the diagrams for the JO100 and HO125 just to see the differences:-
JO100
HO125
