Interesting features on Webster Electric PA

[3choplex]

Someone gave me a Webster Electric TP25-1 PA to salvage.

Mine is all black, but looks like that. Really cool. Too bad the mic transformers are gone.

It's very similar to this, but 6SN7s instead of the 6FQ7s: http://www.angelfire.com/vt/audio/TP45schematic.gif

Some interesting things in there that I haven't seen before:
The

• OT has a negative feedback wire built in.  It makes sense, but I've never seen it.
  It also has a non-adjustable fixed bias supply. Isn't that how Mesa does theirs? Seems a bit silly, but what do I know?
  The Chassis has a really strange layout, presumably to keep it compact. The transformers are to the side of the chassis, the 6L6s are sunken into it at a 45 degree angle, and the plugs for the mic transformers are in a sort of inverted doghouse that juts into the inside of the chassis.

Kooky.

I'm thinking of building something simple with two EF86s set for lower gain than the standard vox way to try to get nice grind without microphonics and crazy noise into two 6L6s. Haven't started planning yet, though, just taking measurements at this point.

[PRR]

AngelFire doesn't like hot-links from other sites. Let's get the pictures here:

[PRR]

> OT has a negative feedback wire built in.  It makes sense, but I've never seen it.

Makes a lot of sense.

That's a LOT of output taps. Do you know what they all do? Are any useful?

When you see super-duper output like that, suspect it may be marketed to "special uses". Like a school with a mile of wiring in the walls. You really want that wiring floating and balanced, but the NFB wants to be chassis/ground referenced. A few turns of skimpy wire is the way to go. Fairly common on PA amps. Using the speaker winding for NFB is a bit cheaper, and also necessary for VERY impressive Damping Factor, but that's a hi-fi fetish.

> It also has a non-adjustable fixed bias supply. Isn't that how Mesa does theirs? Seems a bit silly

If you know your tubes are consistent it works very well. Especially if you bias to the cool side (don't worry over-much about soft-signal rasp).

> I'm thinking of building something simple with two EF86s

I don't see what's wrong with going into the Mixer grid? You have your EF86 Pentode gain-stage, a Volume control, then the ever-popular James tone-stack (here split into two stages for slightly less interaction). Then the mild-gain 6FQ7/6SN7 driver with NFB, and a cathodyne phase-spitter which always works well. ---EDIT...no volume control.....

One design weakness. The common cathode-cap on the two tone-control triode amps MUST be large and healthy. Maybe NBD with the mild 6SN7 plus high-knob loss. I'd be inclined to split pins 3 and 8, use separate 3K+22uFd on each.

Screen resistor is quite large. This may help limit the damage when idiot users abuse the power. If you scale-back the 6k2 to typical g-amp values, you will need more G1 bias but as you say it is un-trimmable. Personally I've had good sound from "large" screen resistor so I'd leave it the way Webster has it.

[3choplex]

Thanks! I'll try those ideas. I hadn't even really considered keeping it stock, but I should at least try it. It does work, at least somewhat.

Another interesting feature--no chassis ground. It's all on a buss. I assume that when I add a 3 prong, I will still want to make earth a chassis ground?

[sluckey]

Another interesting feature--no chassis ground. It's all on a buss. I assume that when I add a 3 prong, I will still want to make earth a chassis ground?

Yes, the green wire in the power cord ties to chassis. You will also want to connect your ground buss to chassis unless there is some electrical reason to not do so.

[3choplex]

I don't see what's wrong with going into the Mixer grid? You have your EF86 Pentode gain-stage, a Volume control, then the ever-popular James tone-stack (here split into two stages for slightly less interaction). Then the mild-gain 6FQ7/6SN7 driver with NFB, and a cathodyne phase-spitter which always works well. ---EDIT...no volume control.....

I could just plug into the phono input, it goes right to the mixer grid and has a volume control--although I'd rather have the volume after the first stage than before it.

[jjasilli]

OT Secondaries.  This Stromberg Carlson schematic may give clues:  http://www.el34world.com/charts/Schematics/files/audio_-_pa/stromberg_carlson_aph1200.pdf  Your amp may have constant voltage outputs, typically 25V, 70V and maybe 115V; and maybe 4, 8 and 16Ω taps also, not to mention a CT on the secondary side.  As PRR says there may 2 sets of identical taps to run 2 lines of speakers in a school.

Mic Input Trannies:  These were optional and probably were never installed.  Hi-impedance mic's were used for announcement purposes.  You could use in-line adapters for lo-impedance mic's.  Probably your amp has 9-pin sockets for plug in mic trannies.  They are sold on eBay: Bogen, RCA, UTC for about $50+. 

Mods.  I'm guessing you have covered trannies with metal leaves.  If so, the good news is that they are full frequency 20 - 20,000Hz.  The bad news is that they are boat anchors, and the full frequency range is not needed for guitar.  You have enough tubes for two channels, or cascading gain stages. These amps are also suitable for hi-fi though you'd need at least a pair, or a PA amp if you have use for one.

[3choplex]

OT Secondaries.  This Stromberg Carlson schematic may give clues:  http://www.el34world.com/charts/Schematics/files/audio_-_pa/stromberg_carlson_aph1200.pdf  Your amp may have constant voltage outputs, typically 25V, 70V and maybe 115V; and maybe 4, 8 and 16Ω taps also, not to mention a CT on the secondary side.  As PRR says there may 2 sets of identical taps to run 2 lines of speakers in a school.

Yes, it has 4, 8 and 16 ohm taps. I need to put in an actual speaker jack, though, I want more convenience than I get from using screws.

Mic Input Trannies:  These were optional and probably were never installed.  Hi-impedance mic's were used for announcement purposes.  You could use in-line adapters for lo-impedance mic's.  Probably your amp has 9-pin sockets for plug in mic trannies.  They are sold on eBay: Bogen, RCA, UTC for about $50+. 

I actually got it because it had the transformers and a buddy out of the country wanted them. I ended up with a free boat anchor. :) I have one of high/low Z adapters, that was how I verified it worked. It's really low volume, though.

Mods.  You have enough tubes for . . . cascading gain stages.

Yeah, that was my thought as well. I'm definitely going to try just plugging into the phono input before I start doing anything else. PRR always seems to know best. 

[eleventeen]

That was on ebay about 10 days ago and I missed it. That very one. It looked pretty nice, but I am one of those who consider what incoming freight costs (not just the item itself) and it was a tiny bit rich for me.

I WILL say that the trannies and overall construction looked pretty good...better than average.

[HotBluePlates]

Another interesting feature--no chassis ground. It's all on a buss. I assume that when I add a 3 prong, I will still want to make earth a chassis ground?

Yes, the green wire in the power cord ties to chassis. You will also want to connect your ground buss to chassis unless there is some electrical reason to not do so.

If every ground in the circuit is truly separate from the chassis (check continuity to verify), then you could add a ground lift if you think you might have a multi-amp setup at some point.

Connect the green wire of your 3-wire power cord to the chassis. Install a SPST switch on the chassis. Get a 51kΩ 1/2w resistor, and a 0.1uF 630v cap (some use higher, like 1kV metallized polyester). Put these in series from the circuit ground to the chassis. Connect the lugs from the switch to either end of the series cap/resistor.

99 times out of 100 you'll have the switch set to short out the cap/resistor, because it will reduce hum/buzz. On the rare occasions where you have a multi-amp setup that would cause a ground loop, you can flip the ground lift to kill the noise, while keeping the chassis bonded to earth.

[Hebert]

AngelFire doesn't like hot-links from other sites. Let's get the pictures here:

hello, I see the schematic you have for this amp with colors labeling the OT wires. I have this OT but do not know which wires are which. do you know which wires are for 4 ohm, 8  ohm and 16 ohm?  thank you

[Hebert]

hello. if you still have this amp I would be really appreciative if you could take a moment to let me know which wires go to 4 ohm, 8 ohm and 16 ohm. I have this transformer but do not know which wires are which. thank you

OT Secondaries.  This Stromberg Carlson schematic may give clues:  http://www.el34world.com/charts/Schematics/files/audio_-_pa/stromberg_carlson_aph1200.pdf  Your amp may have constant voltage outputs, typically 25V, 70V and maybe 115V; and maybe 4, 8 and 16Ω taps also, not to mention a CT on the secondary side.  As PRR says there may 2 sets of identical taps to run 2 lines of speakers in a school.

Yes, it has 4, 8 and 16 ohm taps. I need to put in an actual speaker jack, though, I want more convenience than I get from using screws.

Mic Input Trannies:  These were optional and probably were never installed.  Hi-impedance mic's were used for announcement purposes.  You could use in-line adapters for lo-impedance mic's.  Probably your amp has 9-pin sockets for plug in mic trannies.  They are sold on eBay: Bogen, RCA, UTC for about $50+. 

I actually got it because it had the transformers and a buddy out of the country wanted them. I ended up with a free boat anchor. :) I have one of high/low Z adapters, that was how I verified it worked. It's really low volume, though.

Mods.  You have enough tubes for . . . cascading gain stages.

Yeah, that was my thought as well. I'm definitely going to try just plugging into the phono input before I start doing anything else. PRR always seems to know best. 

[trobbins]

If you measure the turns ratios of all the windings, then knowing the likely primary PP impedance (from 6L6GC datasheet and 30W rating), and knowing it has 4, 8, and 16 ohm taps, you can deduce all the tap ratings.  Have you measured turns ratios before?