18Watt.com

Bieworm wrote: ↑

Tue 06/21/22 4:37 pm

58T100 wrote: ↑

Tue 06/21/22 3:36 pm

Bieworm wrote: ↑

Tue 06/21/22 3:03 pm

The plate voltage will change when you raise the cathode resistor value, so that math won't work like you assume.

My point is just that if my meter is wrong and it is actually near 180ohms, and I measured the voltages with it in there, using 180 in the calculator and those voltages puts it at barely over the line. So I don't know if it is possible that just 106% dissipation would have caused what I saw - not sure what the explanation would be unless that resistor is significantly less than 180. I am assuming the meter is at least measuring voltage correctly though.

Trying to find some loose resistors to test.

Don't worry.. 106% won't fry a healthy tube immediately in a cathode biased amp. That's not the problem if that would be the value. But did you take the voltages with the sale DMM? They wouldn't be correct either if the device is faulty.

This is all super helpful, and you guys are right. I need a new DMM.

I went and borrowed one and the resistor is 177ohm.

I am actually pretty confident that my voltage readings are okay or at least close - on a new AA I get 1.61, on a 9v I get 9.61, and wall was IIRC 117 and the heaters etc. were pretty much on IIRC.

Since I don't have anything else to go on until I restart it (will have to be with a higher resistor if using these tubes - see below), if we assume the voltage was correct (plate to cathode voltage - 343V; 14.2V drop) and use the corrected 177 value, what would be a best guess for what I should swap to to cool it off?

Also, as far as why these tubes aren't handling it if it was only 108%, I found mention in another thread of Amperex Holland tubes being problematic potentially due to not being rated for the voltage. Plus mine are Hammond pulls not NOS. One thing for sure, one is now dead and blew the fuse, and another started glowing pretty bright and humming within a couple of minutes at idle. Here's the thread -

viewtopic.php?t=7774

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Post by phsyconoodler » Sun 01/08/06 4:45 pm

I had the same problem with some NOS tubes.I had to up the cathode rsistor a lot and finally just went with some new EI tubes.The NOS tubes could not handle the voltage above 300v.That is not the case with all NOS EL-84's,just the ones I had.If I use a zener diode to drop the B+ they work fine,but the sound is not the same.Good,but not the same.I wanted to make it work with all EL-84's so i just ended up with new production tubes.They sound phenomenal!
The ones I had trouble with were Amperex."

Unit_1 wrote: ↑

Tue 06/21/22 5:17 pm

JMPGuitars wrote: ↑

Tue 06/21/22 10:57 am

How many of your 180R resistors are measuring 130? If multiple, I would start questioning the accuracy of your meter. I've never seen a new Xicon 180R with that high of a variance.

Every time I start getting crazy results from my dmm I pull the battery and use my *other* meter to check the voltage only to find 7v or something instead of 9v. Replace the battery and back in business. Wondering why they don't have a battery self check, like my MOUSE has one...... lol

I replaced the batteries and got the same result.

New shopping list is growing - different El84s, new DMM, more resistors.

"Only 108%" is a funny way to think of it. The tubes in question probably should be biased under 70% (assuming there isn't something else causing the problem). Good tubes can handle 75 to 85% reliably, but that's not what you're dealing with. I would order values between 200 to 300 ohm at least. I'd probably start with the 220 or 250 and see where that puts you. If you have a 300 to 500 ohm 3W pot, you can dial it in and see what works, then replace it with a resistor.

You can compare your voltages to the charts in our downloads section.

I use an Amprobe 37XR-A for my main DMM. Whatever you get, get a decent brand. You want a reliable DMM.

Thanks,
Josh

JMPGuitars wrote: ↑

Tue 06/21/22 6:26 pm

"Only 108%" is a funny way to think of it. The tubes in question probably should be biased under 70% (assuming there isn't something else causing the problem). Good tubes can handle 75 to 85% reliably, but that's not what you're dealing with. I would order values between 200 to 300 ohm at least. I'd probably start with the 220 or 250 and see where that puts you. If you have a 300 to 500 ohm 3W pot, you can dial it in and see what works, then replace it with a resistor.

You can compare your voltages to the charts in our downloads section.

I use an Amprobe 37XR-A for my main DMM. Whatever you get, get a decent brand. You want a reliable DMM.

Thanks,
Josh

Thanks Josh, I'll start from there. While I meant "only" relative to the 147.5% I really don't have a background to know what they can take. The bias calculator for some reason says 100% is okay but I'll definitely be following the recommendations here instead.

Fuse is blown in the DMM which I guess might fix it, but now I want that Amprobe.

JMPGuitars wrote: ↑

Tue 06/21/22 6:26 pm

The tubes in question probably should be biased under 70%

Only in fixed-biased amplifiers. with cathode-bias, when the amp cranks, the rise in bias voltage will push the power tubes into cutoff on signal peaks, resulting in splattery-sounding distortion
So, I wouldn't advise biasing any less than 80 %, preferably, 85% for optimum tone and tube life.

zaphod_phil wrote: ↑

Thu 06/23/22 6:18 pm

Only in fixed-biased amplifiers. with cathode-bias, when the amp cranks, the rise in bias voltage will push the power tubes into cutoff on signal peaks, resulting in splattery-sounding distortion
So, I wouldn't advise biasing any less than 80 %, preferably, 85% for optimum tone and tube life.

Even with crappy tubes?

JMPGuitars wrote: ↑

Fri 06/24/22 6:47 am

zaphod_phil wrote: ↑

Thu 06/23/22 6:18 pm

Only in fixed-biased amplifiers. with cathode-bias, when the amp cranks, the rise in bias voltage will push the power tubes into cutoff on signal peaks, resulting in splattery-sounding distortion
So, I wouldn't advise biasing any less than 80 %, preferably, 85% for optimum tone and tube life.

Even with crappy tubes?

It is going to end up with good tubes (although opinions may differ from mine on that). At this point I'm just wanting to get it to a reasonable level before putting anything else in. The 220R resistor will be here today finally and the 250 in a day or two. Thanks again for the guidance on that.

I am a little surprised though as Amperex Holland tubes have always seemed to be quite highly thought of. I have to agree I'm not impressed though. I have always had great luck with new testing strong old tubes for 6v6 and 6l6 so I'll try some more types of those. Otherwise I'll grab some of the Mullard reissues most likely. So far I have here or on the way a pair of matched RCA black plates, a pair of matched Sylvanias, and a pair of original Blackburn UK Mullards. They seem way way easier to come by and cheaper than e.g. 6v6.

58T100 wrote: ↑

Fri 06/24/22 2:42 pm

JMPGuitars wrote: ↑

Fri 06/24/22 6:47 am

zaphod_phil wrote: ↑

Thu 06/23/22 6:18 pm

Only in fixed-biased amplifiers. with cathode-bias, when the amp cranks, the rise in bias voltage will push the power tubes into cutoff on signal peaks, resulting in splattery-sounding distortion
So, I wouldn't advise biasing any less than 80 %, preferably, 85% for optimum tone and tube life.

Even with crappy tubes?

It is going to end up with good tubes (although opinions may differ from mine on that). At this point I'm just wanting to get it to a reasonable level before putting anything else in. The 220R resistor will be here today finally and the 250 in a day or two. Thanks again for the guidance on that.

I am a little surprised though as Amperex Holland tubes have always seemed to be quite highly thought of. I have to agree I'm not impressed though. I have always had great luck with new testing strong old tubes for 6v6 and 6l6 so I'll try some more types of those. Otherwise I'll grab some of the Mullard reissues most likely. So far I have here or on the way a pair of matched RCA black plates, a pair of matched Sylvanias, and a pair of original Blackburn UK Mullards. They seem way way easier to come by and cheaper than e.g. 6v6.

I always use NOS Reflector 6P14P tubes in my 18W amps. I think they are awesome and very cheap

Okay I installed the 220R

Just to be 100%, I can use pin #3 for the voltage drop across the resistor or do I need to measure that directly across the resistor itself?

Pin #7 Plate - 358.8V
Pin #3 Cathode - 14.84V


P-C= 344V
Measured 218.5R
Voltage Drop = 14.84V
91.7% Dissipation

I have a very strong urge to bring it up and play through it a bit, but it looks like I need the 250R. Should I tempt it? Feeling like these tubes aren't keepers anyways.

Ended up taking my own advise. For as much as I abused these tubes before, I take back everything bad I said about them. They sound awesome. Love the amp. The trem. is working great, sounds great, love the breakup & dynamics. This is after several hours.

58T100 wrote: ↑

Fri 06/24/22 8:58 pm

Ended up taking my own advise. For as much as I abused these tubes before, I take back everything bad I said about them. They sound awesome. Love the amp. The trem. is working great, sounds great, love the breakup & dynamics. This is after several hours.

They're really great amps with lots of dynamics and complexe tonal structure. For me it all narrows down to my 18W Tremolo TMB when it really matters.

58T100 wrote: ↑

Fri 06/24/22 8:58 pm

Ended up taking my own advise. For as much as I abused these tubes before, I take back everything bad I said about them. They sound awesome. Love the amp. The trem. is working great, sounds great, love the breakup & dynamics. This is after several hours.

Congratulations! I'm glad it worked out. Don't forget to post a demo! I want to hear those tubes too.

Thanks to you both for all the help!

I'll make a demo and post it up once our company leaves.

58T100 wrote: ↑

Sat 06/25/22 10:41 am

Thanks to you both for all the help!

I'll make a demo and post it up once our company leaves.

If you want them to leave sooner, recording a demo might help.

JMPGuitars wrote: ↑

Sat 06/25/22 10:48 am

58T100 wrote: ↑

Sat 06/25/22 10:41 am

Thanks to you both for all the help!

I'll make a demo and post it up once our company leaves.

If you want them to leave sooner, recording a demo might help.

It turned out the husband was quite fond of loud guitar (and my best tequila)

Consider some shrink wrap and/or silicone underneath your V3A grid wire to prevent a short to chassis.

All done with the case (will still get to a demo soon).

Sorry for the delay, work got really busy. I'll post a recording this weekend.

Please pardon the hamfisted playing - I'm woefully out of practice.

I left in the Amperex Holland EL84s (still at 92% with the 220R) and even threw in some Amperex Holland 12AX7 I found in V1 & V3.

Straight in with an A/B pedal and Tremolo switch - starts with the normal channel on 5 and then switches to the tremolo side on 10 then same with tremolo off.

https://youtu.be/e1t_AY24jn4