Greetings friends, I'm back with this Angela Super Single-Ended project amp. If you're unfamiliar, here's the OG schematic.
I built this with a few changes: only one rectifier (5Y3/5AR4), 12AX7 preamp tube, and a power transformer I had on hand, Hammond 276X, 640v@173mA. Unfortunately this PT is over-rated for this circuit, as I'm only pulling about 95mA with a pair of 6V6, the b+ is over 350v and thus too high for 6V6 tubes. Tinkering a bit more led me to try a pair of 5881 tubes for output, and while a 5Y3 rectifier runs 5881 tubes at about 75% of full bias (according to RobRob calc) a 5AR4 rectifier gives perfect voltages to bias the 5881s at 95% per RobRob. Schematic with voltages:
So here's my question: How many watts are those 5881's making? Right now I have a Hammond 125ESE OPT, it's rated at 15w 80mA. Each 5881 is idling at 67mA if I'm reading this thing right. Am I gonna melt something? Would I be better off with a bigger OPT, or a smaller PT?
thanks!
I built this with a few changes: only one rectifier (5Y3/5AR4), 12AX7 preamp tube, and a power transformer I had on hand, Hammond 276X, 640v@173mA. Unfortunately this PT is over-rated for this circuit, as I'm only pulling about 95mA with a pair of 6V6, the b+ is over 350v and thus too high for 6V6 tubes. Tinkering a bit more led me to try a pair of 5881 tubes for output, and while a 5Y3 rectifier runs 5881 tubes at about 75% of full bias (according to RobRob calc) a 5AR4 rectifier gives perfect voltages to bias the 5881s at 95% per RobRob. Schematic with voltages:
So here's my question: How many watts are those 5881's making? Right now I have a Hammond 125ESE OPT, it's rated at 15w 80mA. Each 5881 is idling at 67mA if I'm reading this thing right. Am I gonna melt something? Would I be better off with a bigger OPT, or a smaller PT?
thanks!
dubadub,
Welcome to the Tubes / Valves threads. It is for Hi Fi and Stereo playback amplifiers, preamps, etc.
But since you are already here at Tubes / Valves, I will give some answers to your questions.
(The appropriate place for Guitar Amplifiers would have been Instruments & Amps).
A lot of Guitar amplifiers run very hot. Works real well until you are at a Major Paid Jam Session, and then it catches fire. Ouch!
You have 32V into 250 Ohms, 128mA, 64mA/per 5881.
Without measuring, you can not know how much of the 64mA goes to the plate, and how much of it goes to the screen.
But the plate and screen are at 393V or more (393- 32V = 361V plate to cathode.
So the total dissipation of the plate and screen are 361V x 0.064A = 23 Watts. That is the Maximum plate dissipation specification.
But the old 5881 screens were only rated at 270V, and you have 361V on the screens, 91 Volts over specification.
The outputs are running very hot. Might fail, especially the screen which is actually 397V - 32V = 365V, 4 volts higher than the plate (it is hard on a tube to run the screen voltage higher than the plate).
Then you are passing 128mA through the 80mA max rated output transformer. It is Saturated.
Saturated is one thing, but that badly, even for a guitar amp will change the character of low notes, and mid and high notes.
You need to consider some re-design from what you are proposing.
I have to go offline now; other duties calling.
Welcome to the Tubes / Valves threads. It is for Hi Fi and Stereo playback amplifiers, preamps, etc.
But since you are already here at Tubes / Valves, I will give some answers to your questions.
(The appropriate place for Guitar Amplifiers would have been Instruments & Amps).
A lot of Guitar amplifiers run very hot. Works real well until you are at a Major Paid Jam Session, and then it catches fire. Ouch!
You have 32V into 250 Ohms, 128mA, 64mA/per 5881.
Without measuring, you can not know how much of the 64mA goes to the plate, and how much of it goes to the screen.
But the plate and screen are at 393V or more (393- 32V = 361V plate to cathode.
So the total dissipation of the plate and screen are 361V x 0.064A = 23 Watts. That is the Maximum plate dissipation specification.
But the old 5881 screens were only rated at 270V, and you have 361V on the screens, 91 Volts over specification.
The outputs are running very hot. Might fail, especially the screen which is actually 397V - 32V = 365V, 4 volts higher than the plate (it is hard on a tube to run the screen voltage higher than the plate).
Then you are passing 128mA through the 80mA max rated output transformer. It is Saturated.
Saturated is one thing, but that badly, even for a guitar amp will change the character of low notes, and mid and high notes.
You need to consider some re-design from what you are proposing.
I have to go offline now; other duties calling.
Thank you for the comprehensive reply. I see that I get 64mA/tube by Vk/Kr. Much too much for 125ESE. The old Tung-Sol datasheets spec 450v max for g2, JJ does too. 1.2k resistor doesn't drop enough, guess I'll go 2k. Or maybe some Zeners and a 10uF, a g2 Supply.
I think I need to look at a new PT, possibly the Hammond 272BX, 600V@115mA. Save this 276x for a HI-FI stereo amp.
Thanks again,
Will
I think I need to look at a new PT, possibly the Hammond 272BX, 600V@115mA. Save this 276x for a HI-FI stereo amp.
Thanks again,
Will
As I mentioned, I had that 276x on hand. I didn't realize that I'd get such a high voltage because I wasn't using its full rated output. The recommended Deluxe PT is rated at 660v@138mA or 700v@115mA. But since there aren't any voltages on the OG schematic I didn't realize I was going big until it was built. One of the EH 6V6 I had in there was red plating, the other was ok.
Figuring this out as I go.
Figuring this out as I go.
You can model the PS in Psud2: https://www.duncanamps.com/psud2/
I'd also be searching thru a guitar forum for voltages related to your original schem., so you'll at least have some kind of reference.
jeff
I'd also be searching thru a guitar forum for voltages related to your original schem., so you'll at least have some kind of reference.
jeff
Those Poor 6V6 tubes; I did smell burning Beaver Pelts!
Reduce the B+ input capacitor until you get the B+ voltage you want to use 6V6 tubes.
Start without any input cap, only the rectifier to the choke (directly).
Then add capacitance in small increments, 1uF, 2uF, 4uF.
For fun, wire the 6V6 tubes in Triode wired mode (at the new lower B+ voltage).
Lower power, low distortion at low power, then a little more signal and you get clipping (yea! high distortion).
Paul Reed Smith; Gibson; Fender, or what?
Tube Ratings:
6L6GC: Plate 30 W, Screen 450V and 5 Watts (all other 6L6 types, much lower ratings)
Some 5881: Plate 23 Watts, Screen 450V 3 Watts (other 5881: Screen 270V)
Note: I use both the JJ 6L6GC, and the JJ 5881.
Consider the tradeoffs. The sound you want; the output power you want; the reliability you want.
The mix of the combinations are infinite, but not always easy to achieve.
Any vacuum tube can be abused.
Your Mileage May Vary
Plug and Play; or Plug and Pray?
Have Fun!
Reduce the B+ input capacitor until you get the B+ voltage you want to use 6V6 tubes.
Start without any input cap, only the rectifier to the choke (directly).
Then add capacitance in small increments, 1uF, 2uF, 4uF.
For fun, wire the 6V6 tubes in Triode wired mode (at the new lower B+ voltage).
Lower power, low distortion at low power, then a little more signal and you get clipping (yea! high distortion).
Paul Reed Smith; Gibson; Fender, or what?
Tube Ratings:
6L6GC: Plate 30 W, Screen 450V and 5 Watts (all other 6L6 types, much lower ratings)
Some 5881: Plate 23 Watts, Screen 450V 3 Watts (other 5881: Screen 270V)
Note: I use both the JJ 6L6GC, and the JJ 5881.
Consider the tradeoffs. The sound you want; the output power you want; the reliability you want.
The mix of the combinations are infinite, but not always easy to achieve.
Any vacuum tube can be abused.
Your Mileage May Vary
Plug and Play; or Plug and Pray?
Have Fun!
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