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Old 13th August 2019, 03:47 PM   #11
stephen_keller is offline stephen_keller  United States
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Quote:
Originally Posted by Mastaphat View Post
1. The cathode as well as the plate has a 240v charge. Why would that be?
I assume you are talking about V2A. I'm not sure why Bogen would run that stage in parallel with a fixed resistance. I'm guessing it is an attempt to semi-fix the cathode bias of that stage. The 220K resistor and the 680 ohm resistor (along with the tube an its load resistor) form a divider that sets the cathode voltage of that triode at roughly .8 volts. It is not fixed because it will still vary slightly with the signal swing, but it might be more stable than without the parallel resistance. Seems like a bypass capacitor would have been be more stable choice, but maybe those were too expensive at the time.

Quote:
2. The upper secondary winding of the PT has a lone diode putting off a -23v. Any hints?
That is the negative bias supply for the output tubes.

Stph

Last edited by stephen_keller; 13th August 2019 at 03:54 PM.
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Old 14th August 2019, 12:44 AM   #12
Mastaphat is offline Mastaphat
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Thanks for clearing that up.

Regretably, it'll be another week or more before I have enough time and money to build the light bulb limiter and to get a decent multimeter. So I'll have to quell my excitement for the moment.

It seems that spending ~$50 for a used Fluke is the way to go. So that's my current plan. I have an old analog meter from radio shack but it's been banged up and unused for years in the bottom of a box.

In the meantime, have a 40w Weller soldering iron but only one tip that I've used forever. Will this do or should I plan to invest in my soldering equipment as well? Here is a link to what I currently own:
WELLER Station, Solder, 5-40 W - 5JH84'|'WLC100 - Grainger

The ax-p1 pdf was a good read. I'm eager to further my understanding of amps/circuits so I'm open to suggestions for what I should read and learn from next.
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Old 14th August 2019, 03:12 AM   #13
stephen_keller is offline stephen_keller  United States
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Your Weller looks just like mine. Clean and re-tin the tip if it's not terribly pitted. Replace the tip if it's badly beat up.

I will put together a reading list for you if someone else doesn't beat me to it.

Stph
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Old 14th August 2019, 04:34 AM   #14
PRR is offline PRR  United States
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Quote:
Originally Posted by Mastaphat View Post
The cathode as well as the plate has a 240v charge. Why would that be? ...
240V is going "toward" the cathode, yes. But via a Voltage Divider.

You MUST understand Voltage Dividers before all else!!

The divider is 220,000 and 680. So voltage is reduced more than 200:1. A fine calculation says 0.74V before the 12AX7 comes into play. Not "240V".

We can estimate the voltage dropped across a 12AX7 with 0.74V bias, but current in the tube will lift the 0.74V somewhat. So a bit of circular recursion (try try again) is required. Complicated by 12AX7 being fairly non-linear on the large-scale. I guessed some values from too much experience.

Then I remembered Bogen always posted voltages. While hard to squint in this scan, they agree within a ballpark.
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Old 14th August 2019, 04:47 AM   #15
PRR is offline PRR  United States
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Quote:
Originally Posted by Mastaphat View Post
...The upper secondary winding of the PT has a lone diode putting off a -23v. Any hints?...
Again, Voltage Divider!!

The main B+ is 450V from a Voltage Doubler (which is no real relation to a Voltage Divider). Using a non-doubler rectifier we expect 225V DC. *But* there is a 56k:15k resistor divider, so about 1/4th, so say 55V. But the 39k after the divider is another load. And because the diode conducts in bursts while the 39k sucks all the time, this is not a simple voltage divider. While it can be calculated, the usual design is to find a similar plan, steal it, and then tinker to fit.
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