New Build 5/2025

[WRC34]

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’ the builds by ear, troubleshooting issues and swapping out components until the amp is stable and things sound good. But you’re all making me want to invest in a scope etc to get to the bottom of what I’m making and know exactly what’s going on.

Guess I’ll have to start skipping lunch & save up!

[sluckey]

In lieu of having a scope I use the DMM, as I think many hobbyists do, but it’s good to know the results are inaccurate.

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?

Your DMM measures DC resistance (AKA DCR) accurately. But DCR is not the same as Impedance. You really don't need to get bogged down on speaker impedance. Just take the manufacturer's work that the speaker impedance is what they say it is.

I bought a Celestion Blue especially for my Vox AC-15 and Marshall 18W. I would never play those amps dimed when using that Blue! I also have several other 15 to 20 watt amps. All are capable of destroying that Blue. So, be careful!

[WRC34]

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?

I dunno, you should see the way I cut the hole in the chassis for a “laydown” transformer https://youtu.be/vOXJtgGgJYk?si=0p8M9QryBSm0yIvE

Also, thank you that’s exactly what I’m asking about the Blue. I want to be able to crank the amp confidently, without having to worry. Will stick with the Weber. I’ve been sitting on the Blue for about 4 years, probably best to wait until I can scrape up the dough to get another and load them into a 2x12 to use with an amp like this…looks like I’ll be skipping dinner, too!

[WRC34]

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?

[martin manning]

No, sorry. You can guess based on your B+ and load impedance like this:
P = 2 * (HT-50)^2 / Rload
P = 2 * (282-50)^2 / 8200 = 13.128
See: https://www.valvewizard.co.uk/pp.html Have a read through that article and the one on single ended power stages.

PS: The amp sounds good, and nice job on the video!

[WRC34]

Thx Martin!

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

[WRC34]

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 cathode 296
2 x (311 - 50)^2 = 68,121 % 9200 = 14.8 watts
OR
2 x (296 - 50)^2 = 60, 516 % 9200 = 13 watts










*note - the 50v subtracted from plate voltage
(assumed lowest plate voltage swing?) is just that. assumed. Not actually measured!

[martin manning]

5AR4 & 6F6s
Plate to cathode is 313
2 x (313 - 50)^2 = 69,169 / 9200 = 15 watts

5U4GB & 6F6s
Plate to cathode 296
2 x (296 - 50)^2 = 60, 516 / 9200 = 13 watts

The above, using plate to cathode voltage, is the correct result.

*note - the 50v subtracted from plate voltage
(assumed lowest plate voltage swing?) is just that. assumed. Not actually measured!

That is correct, and this technique estimates theoretical "load line" power. The power delivered to a load will be lower due to losses in the output transformer.

I noticed recently that if I type e.g. 2(296-50)^2/9200= into the reply window, the answer 13.156 is automatically calculated. IDK who is doing this, but thanks!

[pdf64]

More accurate results might be achieved if the calculator I linked previously was used.
Especially if voltages under load are used.
eg run a continuous tone around the onset of clipping through the amp and measure the V DC at the OT CT, at the screen grid HT supply node (other leg of the 4k7 HT dropper) and at the cathode resistors.
Unfortunately the calculator doesn't seem to have 6F6 option available.

[martin manning]

...eg run a continuous tone around the onset of clipping through the amp...

How to determine the onset of clipping without a scope? And if a scope is available, then why not measure power into the load directly?

[lonote]

This thread & info is timely for my own project that I am just getting measurements on, after firing it up for the first time over the weekend, & trying to reconcile expectations vs real numbers.

Not to hijack, but I built a Monarch clone (from TAG thread/resources, https://ampgarage.com/forum/viewtopic.php?f=6&t=30258) but substituted some 6005 power tubes (first build with them) & used the appropriate lower B+. These are described as a lower voltage, small bottle 6V6.

I recently acquired a scope, but have very little experience with one, so that is also a learning curve.

In a nutshell, my numbers; B+, plate voltage, grid voltage (cathode biased), idle plate current, are all close to the PP data sheet example, the biggest differences being a lower 63mA idle current with an 8K primary OT.

Using the scope, I was surprised that the actual measured clean delivered power to an 8 Ohm dummy load (approx 7.5W) was a good bit lower than I had thought it would be, but my expectation was not based on anything other than it being a "6V6-ish" amp. Running my numbers using the formulas referenced earlier in this thread, it seems they are right in the ball park.

6005 Typical PP Operation.jpg

[WRC34]

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 finding a power transformer with a low enough HV secondary that also has a high enough current capability (mA rating) SO…where did you get the PT for your build??

[sluckey]

Look for an old Akaia/Roberts R2R carcase on eBay. I found this one for cheap. It is just what you need...

[lonote]

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 finding a power transformer with a low enough HV secondary that also has a high enough current capability (mA rating) SO…where did you get the PT for your build??

I used an AnTek toroidal for this one.

https://www.antekinc.com/as-05t200-50va ... ansformer/

180V & 200V taps, no CT, 130mA, 2X 2A 6.3

I used the 200V tap with a FWB & I am getting 255V loaded B+ & 6.36VAC on the heaters, 120VAC from the wall.

I initially was going to run the 6005s higher than the data sheet recommends using a 275-0-275 PT I had on hand, but decided to start out on the safe side & remembered I already had the AnTek for a preamp design I was planning. Seems to work well.

[pdf64]

...eg run a continuous tone around the onset of clipping through the amp...

How to determine the onset of clipping without a scope? And if a scope is available, then why not measure power into the load directly?

Good point, the onset of clipping might not be audibly obvious, especially with this type of output stage.
But my thinking is all that's required is to get an rough level for the sagged HT supply nodes.

How about 'turn it up to maximum, such that ghe amp is clearly overdriving, then back off until it's just starting to overdrive'?

The speaker used for this would need to be in a different room, or be surrounded by stuff to damp the sound level, eg thick cushions over any cab opening.