Tube Amp Archive

Martin, you said: “Fooling around with the 6F6 data sheet and scaling things (data sheet is at 250V Vg2, where you are at 232) I believe your 9k2 load line is crossing the Vg1=0 grid curve at around 100V, and you are only getting around 6.5W output power. In theory, at the same voltages, a 12k load ...

I have been infinitely curious about these 7pin beauties - 6005 is synonymous with 6AQ5 if I’m not mistaken. There’s enough testimony out there stating how glorious these sound for me to have bought some 7 pin sockets and a pair of 6AQ5s but the train always stops there as I have had difficulty find...

Using the formula Martin suggested from Merlin’s website, I got this: with 5AR4 & 6F6s Plate voltage measured 331 from plate to ground. Plate to cathode is 313 2 x (331 - 50)^2 = 78,961% 9200 = 17 watts OR 2 x (313 - 50)^2 = 69,169 % 9200 = 15 watts with 5U4GB & 6F6s Plate voltage 311 Plate to catho...

Thx Martin!

I’m going to take some more readings tonight. Will report back soon

martin manning wrote: ↑Sun May 25, 2025 4:32 pm Plate dissipation is not output power, it's about the severity of the operating conditions for the tube. You need a scope and a dummy load to measure output power.

So, is there no way to use the math and readings I took to discern output power?

It's not that the results are inaccurate. You're just using the wrong tool for the task. You would not use a screwdriver to cut a board, would you? :mrgreen: I dunno, you should see the way I cut the hole in the chassis for a “laydown” transformer ;) https://youtu.be/vOXJtgGgJYk?si=0p8M9QryBSm0yIvE...

Pete, did you find this on Bunker of Doom?? I have probably made 30-40 guitar amps over the past 12 years or so. While only a couple have been clones, they generally have followed along the design guidelines of the 5F1, 5E3, 18 watt Marshall, AC30 and JTM45 / Fender Bassman. I have always ‘tuned’ th...

Sluckey, Man, I am getting some heavy hitters chiming in here! Martin, Pete and now you. Legends. Thanks to you all for keying in. I didn’t know about the way to find screen current, very cool. As a way to determine what’s happening where and to find out whether I’m operating the tube within its lim...

Martin, I’m using a DMM set to ‘ohms’ haha. 8 ohms is the spec determined by Weber. I did not measure the speaker itself. The Jensen though, being old and whatnot, I did measure prior to installation and it read around 6.4 ohms I think. Close to 7. How else can I determine the load? Clip the meter o...

Hi Pete, The OT is a Classictone 40-18037 18 watt Marshall type. The spec sheet shows the reflected impedance to be 9.2K with an 8 ohm load (the Weber currently in there is 8 ohm) and 8.2K with a 7 ohm load and the Jensen was closer to 7 ohms which is what I used to obtain the readings posted. Even ...

But wait…the screens are still dissipating…so while plate current is more accurate by subtracting the suggested percentages the PT still has to handle the plate and screen current, right? 🤔 I did a test this afternoon post re-solder and everything sounded good. No hiccups. The Jensen speaker sounded...

I love it! So, based on these numbers: with 5V4 rectifier & 6V6Gs Plate Voltage 301 Screen Voltage 276 Plate -> Cathode 282 Cathode -> Ground 18.1 / 19.4 tube 1 18.1 % 496 = .0365 = 36mA .0365 x 282 ‎ = 10.293 watts tube 2 19.4 % 496 = .0391 = 39mA .0391 x 282 ‎ = 11.0262 watts 21.3 watts total diss...

Thanks Steve!

You mean plate to cathode voltage? In my figuring this is what I did:

Cathode to ground voltage divided by cathode resistor value (496R)
Multiplied that figure by plate to cathode voltage to determine plate dissipation.

Is this right?

Holy smokes…it was used at a rehearsal today and after about 40 minutes it…stopped working. The fuse hadn’t blown and the rectifier was still “lighting up” and producing DC voltage, but the filaments on the 6.3V tubes were all dark. No sound, obviously. I started to calculate a doom hypothesis - it ...

Hi friends, long time no see. Recently finished an amp and wanted to share here. It’s in a 5E3 size speaker cab and is currently running a ‘61 Jensen P12-N. The circuit bears some similarities with old Fender tweed amps like 5F2 and 5E3, so I’m posting it here but it is not a Fender or clone. I used...