trinityamps.com

Hi all, I've just completed a Tc-15 as my 2nd amp build and I've got a few questions to pose to the knowledgable folks that frequent this forum.

The first (and most important): When I turn on the amp (standby still on) one of the el84 tubes flashes brightly from the filaments for half a second before settling into the regular orange glow. I have switched on the B+ for a few minutes and all parts of the amp seem to work so I don't know if I should be worried about the tube blowing up or is this normal. Unfortunately, this has kept me from putting the amp through it's paces (which kind of sucks when I've just finished such a large project)

Second: How quiet should the amp go? As I turn the volume on either of the preamps it goes from nothing to pretty loud in a small movement (no real bedroom level) I found that with the master volume down I got some weird fizzyness to the sound.

Third: How much hiss/hum is normal? with the master volume bypassed I can only turn the preamp to about 9:00 before inducing quite a bit of noise without guitar plugged in.

I guess I'm a little paranoid since i've spent a lot of time on this amp and as a challenge to myself I re-designed the layout and built it on a turret board that I built.

By the way, I've learned a lot by searching around this forum. Thanks for all your intelligent posts that I've read and learned from.

Oh, by the way for the few minutes I had it on the amp sounded great!

Brandon

What make of tubes are you using?
There are a number of makes (NOS Mullards for one, don't know about new manufacturers) that flash on initial power up.

There is a noise floor to tube amps, sounds like "white noise", but this should be unnoticable as soon as you start playing.

Also the noise should drop considerably with the guitar plugged in and you touch the strings, the guitar is then grounding you and the "noise floor" drops, a body acts like an antenna and increases the noise until you ground yourself by touching the strings.

I'm sure you'll get additional / better explanations here soon.

Thanks JCNY, your explanation on the noise is kind of what I expected. Since my last post I got up the nerve to turn the amp on long enough to take some voltage readings on the tubes and found that I am well over voltage in some places while others are very close. I'll list my findings here with the voltage specified in the build guide in brackets:

AC mains voltage is 120VAC (123VAC)
B+ VDC with tubes is 395 (355VDC) I'm not sure how I am getting an extra 40V here

V1 (12ax7) pin 1&6 124 (130) pin 3&8 1.4 (1.21)
V2 (12ax7) pin 1 166 (159) pin 3 1.4 (1.27) pin 6 284 (197) pin 8 169 (160)
V3 (12ax7) pin 1 250 (190) pin 3 73 (61) pin 6 245 (231) pin 8 73 (61)
V4 (el84) pin 3 8.5 (5.9) pin 7 389 (298)
v5 (el84) same as v4
V6 (5ar4) pin 8 395 (298)
V7 (ef86) pin 1 66 (56) pin 3 1.7 (1.5) pin 6 111 (90) pin 8 1.7 (1.5)

Any ideas? Obviously the high voltage on V4, V5, and V3 concern me.

Brandon

Post some pics of your build.

_________________
http://pdfelectronics.com

OK, here's some pics. The board looks different but really things are just sort of flipped around and the Filter cap for V7 has been moved to the end of the turret board. Any critiques that will help me improve my work are welcome!

Some added info:

I pulled the tubes out to start my measurements from the power transformer. I found that the primary is right on at 123VAC and the low voltage taps are right on at 5VAC and 6.3ishVAC but the high voltage tap is not 290VAC but rather 325VAC. During my initial startup I though maybe that was okay but the combined voltage is 650VAC compared to the spec'd 590VAC.

So what could be up? I'm pretty sure I wired the primaries correctly (black and black/red hot, white and black/white neutral) Could this be a problem with the transformer itself or is it normal for the voltage to be over without the rectifier installed. Help?

Brandon

123 is a bit high to start

Put all the tubes in, what is the HV on the secondary?

_________________
http://pdfelectronics.com

I have put the tubes back in. Putting the tubes in did not change the high voltage AC off the PT. Here's what I found:

With the rectifier tube only, the rectified voltage on pin 8 is about 290V DC (good). I read the B+ with standby on at 435V DC whic compares well to the builder's guide which says it should be 440V without tubes installed. I'm curious why the voltage goes up when you switch on the standby. If anyone knows please fill me in.

With all tubes in and the amp powered up, standby switched to on the B+ is at 390V which is about 40V higher than the 355 suggested in the builders guide. All the pin voltages on the tubes are as I posted before.

Upon searching I found another poster who had similar voltages to mine and Stephen posted this reply



So is this considered within range?

Brandon

Looking through the TC15 docs I found these posted and they seem to jive really close to what I have. I've been going off the chart in the builder's guide and the layout. Is there a reason why there is such a difference between all these?

Is the amp on Standby? If it is, take it out of Standby and repeat your measurements.

_________________
Great tones come in small glass jars!

Sorry the way I wrote that was misleading. All of those measurements are taken with the amp not in standby. So B+ voltage is 395VDC with the amp fully on.

To account for varying voltages, we spec'd the PT at 117 / 290. 123 is 6 VAC or about 5% high, so expect the VAC to be 5% high, and B+ will be high then too. Although, not 395, I would have expected about 365.
check your resistors to all the 22uf caps, and assuming you used 32/32 / 1K5 on the first filter as per our schematic.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

hmm, I checked and it seems that all plate resistors are 22K (or very close) and the 1k5 on the cap can is spot on as well. Any other ideas? I'm not really sure what else can affect the overall voltage. Should I be exploring the option of adding something to drop the voltage or is something definetly wrong? The amp sounds sweet by the way!

If this is OFF StandBy, then possibly, your tubes are very efficient and not drawing down the B+. Still seems larger than I would expect tho in this case.

Confirm if SB On or Off?

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Stephen, these measurements are all with the standby switched off, as in the amp is in a playable state. When the standby is switched on I am getting the correct 298VDC on pin 8 of the rectifier. The tubes are all JJ's except the rectifier which is a sovtek. Is it safe to run the el84s at this high of a voltage? Could one of the filter capacitors be the wrong value? I don't have a way of testing capacitance.

Brandon

I dont think a cap would do that. We have not used the 5AR4 Sovtek rectifier. It might be more efficient. You can easily drop the voltage 20V with a 5W Zener similar to how we do it in the TRIWATT.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Cool. Does this just involve installing a zener diode backwards in the HV line? Also, does it matter where it goes and will it mess up the voltages on my preamp tubes? I'm curious, what is considered ideal for plate voltage on EL84s? There sure seems to be a wide array of opinions on this and I'd like to know what to shoot for. Well, I think I've used up more than my limit of questions for one post.

First it might be a good idea to measure how much the EL84s are dissipating. In this type of va intage-Voxish cathode-biased amp, they should be dissipating close to 100% of their rated 12W max. If they're dissipating much less than that, you will need them biased warmer, so they draw more current and pull down the B+ more.

You can stick some zeners in series in the B+ line after the first filter cap. The zeners must be reverse biased, and their total value must equal the amount you need dropped. As you probably have quite a few volts to drop, splitting up the job between several zeners will improve reliability, and also make it easier to adjust the amount of voltage you're dropping. I suggest getting yourself several 6V 5W zeners and connecting them in series on a piece of tag board. Leave the leads as long as possible to help dissipate heat, and don't even think of sleeving them. Then adjust the number of zeners to get the voltage you're looking for. Bear in mind that the builders guide voltages appear to be abnormally low, and a B+ of around 340V would have the El84s running closer to their sweet spot.

When you lower the voltage, the power tubes will also run a bit cooler, so you will need to measure an re-calculate their dissipation and possibly adjust the bias resistor value. That in turn may make the B+ rise a little again, and so you might just need to add in an additional zener. Or then again, you might not.

_________________
Great tones come in small glass jars!

Ok, I worked out the dissipation at 11 watts, here's how I did it:

Voltage across cathode resistor = 8.35v
resistance of cathode resistor = 128 ohms

cathode current: 8.35/128 = 0.033

Voltage acroos screen resistor = 0.30v
resistance of screen resistor = 99 ohms

screen current: 0.30/99 = 0.003

Anode current: 0.033-0.003 = 0.03

Anode voltage = 381v
cathode voltage =8.45v

381-8.45 = 372v

372v X 0.03 = 11 watts of dissispation (If I infact followed the steps right this is my first time)

So if I'm within the max dissipation and I'm happy with the sound of the amp is it ok to leave the voltage as is or will it cause premature tube failure? I know that the specified max anode voltage for el84s is only 300v but have also heard of guitar amps with 400v and more. If I knew that my tubes would be alright I would just as soon leave things as is because the amp sounds ridiculously good.

You should be fine with that - and yiu now have a TC-20 . Voltage on it's own doesn't wear out tubes - dissipation does. Although when you go way too high with voltage, you get arc-over, which will sound like crackling and popping, and that will instead destroy good tubes pretty quickly. But it doesn't normally happen with EL84s at 400V, and if it was happening, you would already know about it. Just as one further check, play the amp cranked in a dark room and check for screen grids glowing orange on peaks. If they do that, then put in bigger value screen grid resistors to help limit current peaks.

_________________
Great tones come in small glass jars!