Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: vampwizzard on August 02, 2017, 10:41:14 pm
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Hello!
Im taking a later model AO-29 PT/OT and creating a tremolux 5E9 circuit. I reused one of the turret boards, had some fun getting everything to fit in and on the chassis. But im there.
Circuit-wise, here is what im looking at. AO-29:
(http://captain-foldback.com/Hammond_sub/schematics/M3_late.gif)
and the 5E9:
(http://www.thevintagesound.com/ffg/schem/tremolux_5e9-a_schem.gif)
Im looking at a B+ voltage on pin 8 of the rectifier of 325V. So im way under the 5E9's output section voltage but it obviously it pairs well to the AO29 OT. It doesn't pair to the rest of the circuit.
First up, if Im OK with a 250ohm cathode bias resistor and 320V B2+ voltage to the plates of the 6V6, do i need the first voltage dropping resistor? I can replace it with bus for the node. Should I bypass the second filter cap in that case? If you look at this set of voltages off the very similar 5E3 circuit youll see my 320V B1,B2+ Voltage would then allow me a normalish B3+ voltage.
(https://robrobinette.com/images/Guitar/5E3P_Build/5E3_Voltages.png)
If i combine my nodes there, sharing a B1 and B2 voltage, is there anything else I should be wary of while turning on and testing the amp?
Thanks folks!
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There's nothing wrong with 305V on 6V6... this is the recommended condition.
Fender went further. I am not convinced that is better except in racing.
There's nothing really wrong with the Hammond power supply, and I would lean more to it than Fender. Much as I respect Fender, Hammond had a better ear.
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Well, im ok circuit wise and knowledge wise matching the output transformer to the tubes and the B1+ voltage. My question was more about the first voltage dropping resistor and if I should leave it out and share what was B2 with my lower-than-stock B1 voltage.
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Rather than using that 5E3 schematic for reference voltages use this 5E9A schematic/layout. Voltages are on the layout page...
http://el34world.com/charts/Schematics/files/fender/Fender_tremolux_5e9a_schem.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_tremolux_5e9a_schem.pdf)
You have 325V right at the rectifier pin 8. I would expect more voltage from that AO-29 PT. Do you have the filter caps connected while making this measurement? If not, then the B+ will be low until the caps are connected. You can increase your B+ by using a 5U4 rather than a 5Y3. Increase even more by using two 1N4007s.
Is this amp completed and working? If so, pull the 6V6s. How much voltage do you have at the rectifier pin 8 now?
I would not remove the first dropping resistor because the 6V6s have no dedicated screen resistors. You can lower the value to approx. 1K to increase the downstream B+ node voltages. I would install 470Ω/3W screen resistors on each tube socket if you decrease the value of that first dropping resistor. Connect each resistor between pins 4 and 6 and connect pin 6 to B+ node 2.
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I was looking for a sanity check before wiring the power supply to the rest of the circuit to begin troubleshooting. The 325V is rectified but without the filter caps. I forgot about the bump in no load voltage with a filter cap post rectification.. thats the problem with only designing AC power equipment :laugh:
Ill wire the rest of the circuit up and post some node voltages later on. Thank you all for the help thusfar!
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I was looking for a sanity check before wiring the power supply to the rest of the circuit to begin troubleshooting. The 325V is rectified but without the filter caps. I forgot about the bump in no load voltage with a filter cap post rectification.. thats the problem with only designing AC power equipment :laugh:
Ill wire the rest of the circuit up and post some node voltages later on. Thank you all for the help thusfar!
Don't forget to wire the amp with a three prong cord. and get rid of the death cap.
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Amen. What got me into the idea of building a full amp was doing the power cord conversions on some vintage stuff and troubleshooting other people's mods. They are phenomenal things. I'll post more later tonight!
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Update, with voltages
I finalized my wiring and took some measurements. Preamp tubes are installed and are in very tight.. its incredibly difficult to remove them so im leaving them in. 6V6's are not in the circuit. Im using the floating AC filament wiring style, jumpering the filmanets together and adding power at the indicator bulb. Everything seems normal there.
All Voltages DC unless otherwise specified.
B1: 480.4
B2: 472.6
B3: 463.4
Preamp Tubes:
V1 AY7
1:0.64
2:0.31
3:-.1
4:
5: 7.0VAC to pin 9
6:0.72
7:0.32
8:-.1
9:7.0VAC to pin4/5
V2 AX7
1:-0.61
2:0.63
3:-0.07
4:
5: 7.0VAC to pin 9
6:0.7
7:0.3
8:-0.1
9:7.0VAC to pin4/5
V3 AX7 (Tremolo)
1:0.46
2:0.52
3:-0.63
4:
5:7.0VAC to pin 9
6:0.51
7:0.63
8:1.6mV
9:7.0VAC to pin4/5
V4 6V6
1: N/A
2: 7.0VAC to pin 7
3: 485.2
4: 0
5: 0
6:N/A
7: 7.0VAC to pin 2
8: 0
V5 6V6
1: N/A
2: 7.1VAC to pin 7
3: 485.7
4: 0
5: 0
6: N/A
7: 7.1VAC to pin 2
8: 0
V6 5Y3
1: N/A
2: 480.7
3:N/A
4:346 AC
5:N/A
6:345 AC
7:N/A
8:480.1
The B1+ voltage makes more sense. I feel like the voltage drop on the 5K resistor to B2 should reduce the B2 voltage more than it does.. anyone else agree? The Ao-29 schematic says a 30ohm resistor should get the voltage down to 320, albeit with a 5U4 tube. Any thoughts on what I should be looking for? Are these voltages normal or is something amiss?
Thanks!
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Are these voltages normal or is something amiss?
A lot of stuff is amiss!. You have no voltages on V1, V2, and V3. You also have no screen voltage on the big tubes.
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Yeah.. I missed some wiring that i highlighted on my layout that didnt cross reference to the schematic. Thats incredibly embarrassing.
Fixed what I was missing, remeasured voltages. The trem tube (v3) Its hard to get a reading.. ive got oscillation on every DC pin on that tube most hovering around 200VDC.
Here are the new voltages:
All Voltages DC unless otherwise specified.
B1: 467
B2: 438
B3: 330
Preamp Tubes:
V1 AY7
1:171
2:0
3:2.7
4:
5: 7.0VAC to pin 9
6:164
7:0*
8:0*
9:7.0VAC to pin4/5
V2 AX7
1:171
2:0
3:2.7
4:
5: 7.0VAC to pin 9
6:175
7:0
8:2
9:7.0VAC to pin4/5
V3 AX7 (Tremolo).. ive got oscillation on these pins, very difficult to measure. im worried about this (not sure if I should be.. we are making an oscillator) but I consider that a secondary problem.
V4 6V6
1: N/A
2: 7.0VAC to pin 7
3: 471
4: 442
5: 0
6:N/A
7: 7.0VAC to pin 2
8: 0
V5 6V6
1: N/A
2: 7.1VAC to pin 7
3: 469
4: 439
5: 0
6: N/A
7: 7.1VAC to pin 2
8: 0
V6 5Y3
1: N/A
2: 465
3:N/A
4:348 AC
5:N/A
6:348 AC
7:N/A
8:467
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(Tremolo).. ive got oscillation
That's a good thing, activate the FS to turn OFF trem, then measure.
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> V4 6V6
> 5: 0
> 8: 0
We expect either +24V on pin 8 or -24V on pin 5.
Zero-zero implies infinite current and quick melt-down.
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https://youtu.be/acFs3iYGt0c (https://youtu.be/acFs3iYGt0c)
Got it running! Finished the amp around May. Sorry for taking so long to get content updated. Have some new stuff coming.. a restoration and some new conversions on other hammond platforms.
Overall very happy with the tone. Was too bassy with the higher current rated transformers vs the 5E3 underpowered iron. halved the input caps but would probably lower again. Made a potato box looking cab with a 15inch eminence speaker. Demo above is with a 12 inch red fang and no pedals.