Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DummyLoad on February 12, 2011, 08:58:39 pm
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i recently acquired a 4 x 6L6G baldwin mono organ amp - i think it's a model # 5709. it has 2 x 5U4G rectos, 2 x 4uF 600V filters and a big ass choke. buttery and i reverse engineered it and the schematic of just the PS has attached.
anyone seen this arrangement before? is this considered a choke loaded PS? it would seem as so, since otherwise the 6L6Gs would be seeing well over 500V to the plates. has
on a footnote, i believe this to be a ~~50W amp - 2 pairs of 6L6G cathode biased and bypassed.
the phase inverter is a 12XA7 Paraphase.
amp was constructed around 1958 - at lease that's the date code on many of the parts.
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Oh, it's cap-input, but the cap is WAY small. You expect 40u-100u here, they have 4u. The ripple-dips are 350V, from a 550V peak. Then the L-C filter takes the average, around 400V.
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I found ye olde Baldwin schematic. :smiley: There are no voltages posted but I saw a similar schematic that said 305V on the plates. that's 150V less than what we measured unloaded. The circuit ain't clicking for me yet.
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ok, i think i may understand it... hoping for clarification tho.
:smiley:
as we all know, inductors oppose change in current flow. and caps oppose changes in voltage. ya, ya, call me captain obvious, but bear with me... as the first diode charges C, the CT is also charging the inductor, as the power through the diode swings negative and the diode blocks, the second diode is swinging positive and charging the cap and the choke. the choke acts like a current pump for C during both 1/2 cycles or 120Hz. that's why the CT is placed between C and L. in other words, the center tap keeps L charged and L pumps current into C and as the V across C falls C needs more current to oppose change, so our inductor will discharge. it's ingenious, the vacuum diodes see a capacitive load and inductor is "hidden". you can use low values for C since the inductor acts like a current pump, the effective capacitance is larger.
it's ingenious if caps are expensive that is... however today, even 600V caps are much less costly than big iron, and caps weigh much less. even now 600V caps are getting harder to find, but we can series stack them and it's still much less moolah than big iron.
by the way buttery, the OT is fried - 1/2 primary is open. it came that way - i'll explain l8r. :sad:
sometimes, in darkness there is light...i think we may have found replacement iron for glo-champ. the 5V ran 2 x 5U4G - that's 6A. 4E27 takes 7A but i'd bet baldwin iron is overrated. this will liberate design of one more piece of iron. i'm going to have to recalculate B+ tho looks like it'll be closer to 600V, so glo champ will need new bias resistor - but then again, maybe not. :wink:
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I found ye olde Baldwin schematic.
thnx buttery. :smiley:
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> the CT is also charging the inductor
Unimportant.
You have second cap, one end tied to circuit common.
You have PT, 5U4, and first cap, making rough DC, but not tied to ground in any way.
You must run two connections from first cap to second cap to complete a circuit.
In one of those you want a choke.
Since the PT HV winding is not grounded, you have the option of putting the choke in the "ground" leg.
The circuit works the SAME.
There is a belief that putting the choke in the groundier side causes less stress on choke insulation. (The difference is slim.)
It's an obvious way to do this, cited in many ham manuals.
> 150V less than what we measured unloaded
With "ample" capacitance, 40u-100u for a large power amp, the voltage changes little from no-load to nominal load.
With LOW capacitance, the ripple is small at no-load but HUGE at full load, the average DC falls like a 4uFd cap. Conversely it un-sags at no-load much more than you are used to with "ample" cap filters.
And yes, this does mean 600V caps to stand the cold-start surge, when 5U4s comes up a little faster than 6L6es, even though the operating voltage may be 300V-400V.
> ingenious if caps are expensive
Baldwin was about IRON. Locomotives, pianos (frames). Caps were funny-parts but a choke they could understand, probably build in-house.
> caps weigh much less
This was an ORGAN. And surely Fixed Install. In a stone church which should stand until Gabriel comes on stage with his angelic choir. Between over-walnutted console and pedal-bottom woofers, what's another few pounds of choke? It's not going anywhere. (OK, it did; neighborhoods and fashions change, churches play rock-music, sell-off the catholic/episcopist trappings, or the church gets blown-down for shopping or freeway bypass.)
Also if done right, the choke and oil-caps will live 50-100 years in 24/7 service, while electrolytics don't last as long as a good pastor. It's bad form for the organ to buzz-out for a mass or wedding a few years down the road. (But the "Jons" diagram appears to use electrolytics in lower stages.)
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thanks PRR - the light bulb is glowing now...about all of 2 watts now.
they are electrolytic caps. these still seem good - discharge time was about 15 secs. amp is scrap except for PT and inductor - sad; i was hoping to get running and acquire another for hi-fi, - seen 'em on fleabay on a few occasions.