Converting AO-43 into 1624t

[cobalt]

Hello All!

I am about to start this build. I do however have a couple questions......I am using this schemo:
http://el34world.com/charts/Schematics/files/supro/supro_1624t_scematic_263.pdf

1. First the error, power tube cathode bias resistor is 250 Ohm NOT 250K right?

2. What is the switch pictured between power tube control grid feeds? Is that connected to tremolo speed control?

3. I know I can add another power supply node for screen grids(20uF 450V), and add 1K to each screen grid(protection and stability), BUT... would the ruin the original "vibe" of the amp?

4.I do have a voltage problem. The PT is putting out 760+ Volts(unloaded), so will be looking to drop some. Perhaps you have some suggestions....... maybe recto protection style(before recto) or brute force(after recto). Either way is going to make current borderline.

5. Will be using NOS 6CZ5 tubes. Have a bunch, both small and tall. I figure they will be better than new production 6973, by a long shot. Any concerns here?

Thank you for reading, thanks a BUNCH for any advise you can give.

weirddave


{EDIT: fixed munged URL --PRR}

[sluckey]

1. First the error, power tube cathode bias resistor is 250 Ohm NOT 250K right?

Correct

2. What is the switch pictured between power tube control grid feeds?

Mute. Silvertone called it a Standby switch on the 1484.

3. ... would the ruin the original "vibe" of the amp?

Don't know. Try it and see.

4.I do have a voltage problem. The PT is putting out 760+ Volts(unloaded), so will be looking to drop some.

Probably too high.

5. Will be using NOS 6CZ5 tubes.

Sorry, not familiar with that tube.

[PRR]

> not familiar with that tube

6CZ5 is a TV era "6V6". Mini 9-pin bottle, higher voltage rating. Marketed for TV V-sweep (hence the higher voltage rating) but also has audio suggested conditions (on the Sylvania sheet).

Is probably NOT internally built just-like a 1938 Ken-Rad 6V6; OTOH was probably tweaked to be no-surprise in any 6V6 circuit.

[DummyLoad]

> not familiar with that tube

6CZ5 is a TV era "6V6". Mini 9-pin bottle, higher voltage rating. Marketed for TV V-sweep (hence the higher voltage rating) but also has audio suggested conditions (on the Sylvania sheet).

Is probably NOT internally built just-like a 1938 Ken-Rad 6V6; OTOH was probably tweaked to be no-surprise in any 6V6 circuit.

6973 seems like a hopped up 6CZ5. i agree with sluckey, 700V+ PT will cook them. 620VCT is more like it with 5Y3 VTR.

wards airline 9023A is the same amp. schema attached - looking at the wards schematic, it seems like you want 350V B+, so your PT will need to be around 150mA @ around 620VCT with a 5Y3GT rect. schematic shows 23V cathode/250R shared cathode R = ~92mA plate and G2 current at idle. in this case i'd probably use a hammond 260J
with this OT

--pete

[terminalgs]

5. Will be using NOS 6CZ5 tubes. Have a bunch, both small and tall. I figure they will be better than new production 6973, by a long shot. Any concerns here?

6CZ5 design center voltage is 350v,  for the 6973, design maximum is  440v. I once tried to run a 6CZ5 at 420v. it did not like it! 


Magnatone 280s came with 6CZ5's initially and moved to the 6973 with the 280A.  The schematic for a 280A lists the 6973 and 6CZ5 as being interchangeable* , and 375V at the plate.  Also note, the 280/280A is fixed bias.


(edit: I say interchangeable* because Magna wired the socket to accept either tube,  the tubes were interchangeable in the 280A, not interchangeable in general terms)

[DummyLoad]

6CZ5 design center voltage is 350v,  for the 6973, design maximum is  440v. I once tried to run a 6CZ5 at 420v. it did not like it!

this amp is 350V B+, so shouldn't be a problem with using 6CZ5 if B+ is kept within that realm... 

just a guess, but max. Va rating variance might be reason for the minor difference in 6CZ5 and 6973 basing relating to G2, specifically pin 8: otherwise, with all other electrical parameter, and structurally they appear to be identical.

--pete

[drew]

4.I do have a voltage problem. The PT is putting out 760+ Volts(unloaded),

Are you sure about that?  Isn't this the one that the Hammond schematic says is 315-0-315?

[cobalt]

Hello,


In regards to voltage, I got two surprises when checking PT secondary, 1,(pins 3&5 on recto socket)763V!!!!, 2, my DMM DIDN'T pop!(700Vmax)

Yea, pretty sure about voltage. Was thinking of trying 5R4GXX, might drop a few. Now I have to get one!

weirddave

[kagliostro]

2. What is the switch pictured between power tube control grid feeds?

Mute. Silvertone called it a Standby switch on the 1484.

This "cross line" mute is a good choice or is better to ground the grids of the final tubes ?

Thanks

K

[sluckey]

Crossline switch only needs to be a SPST and will work on fixed bias or cathode bias output stage. If the output stage is not exactly balanced there may be a little bleed thru sound when muted. Enough to be heard but not enough to be a problem.

Grounding the grids would require a DPST switch and will work well on this cathode biased amp. Amp would be absolutely quiet with this method.

[kagliostro]

Thanks Steve

Franco

[jojokeo]

I agree with Drew, AO-43's don't have that high of voltage. Something's amiss here (like incorrectly wiring (FWB?) and/or measuring from both outer legs?)

[sluckey]

The PT is putting out 760+ Volts(unloaded),

(pins 3&5 on recto socket)763V!!!!,

I believe Cobalt is measuring AC voltage across the entire HT winding, not one leg to ground and not DC volts. So, 760 would equate to 380-0-380 which is only 20% higher than the schematic's claim of 315-0-315.

Several years ago when I was following the 18watt.com forum, converting AO-43 amps to 18 watt based amps was very popular. A common complaint was 'my B+ is too high'.