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PostPosted: Sat Dec 07, 2019 3:32 pm 
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mitch m wrote:
The voltage at the cathodes of the preamp tubes is very low, around 1 or 2 volts. So electrolytic capacitors rated at 50 volts would be suitable.


Ahh, yes, now why didn't I realize that? :) Typical electrolytics are polarized, I assume the + side goes to the tube and the - side goes to ground, right?


mitch m wrote:
The way I read it, the mods suggested specify what goes on the cathode of V1.
...


Thank you again, what you wrote is what I thought was the case, just wanted to make sure. Stephen needs to get you on Trinity's payroll for all the help you've given me!


mitch m wrote:
The rule of thumb with tube amps is the voltage can vary + or - 10%. If it's 20% over at V4 pin 6 that's starting to get on the high side. The voltage at pin 1 of V4 is good so it's just variance with the tube and not a problem with your circuit. You could try a different 12AT7 there if it bothers you, but the higher voltage isn't exceeding the tube's specs so if it sounds good I wouldn't worry about it.


Awesome. I choose not to worry about it!


mitch m wrote:
Thanks for the compliment about my DR504. It was actually my second tube amp project. The first one was a Trinity TC-15 I built about a year earlier, but I had been building electronic projects for over 30 years by that time. The assembly work for the original Hiwatt amps was contracted out to an outfit in England that also did work for the British Navy. The wiring was very neat and close to military specifications. I tried to emulate that as closely as possible when I built mine. Also I started with a fairly big chassis, like the originals had, so it was easier to keep the wiring from looking cluttered.


Your wiring is a work of art.

Ah, I just figured out my excuse: if I wired my amps that neatly, it would distract from playing, because I'd just want to look at the neat work. So I did less tidy wiring to minimize distractions! ;)

Do you still have and play the Trinity TC-15? Based on the description, that should be very similar to my Dizzy30. Curious what your sonic impressions of the TC-15 are, and how it compares to your Hiwatt-style amps. I like my Triwatt, no doubt, but I feel there is something special about the EF86 channel on my C-30 clone.

By the way, I was playing on the Triwatt last night. I was still at "polite" volume, but I opened up the overdrive channel. Using the "link" input, with the overdrive engaged, literally gives three knobs just on preamp gain tweaking and tone shaping. Anyway, the dynamics of this amp allowed me to get a setting with that overdrive channel where the general tone was almost perfectly clean, but I could get some noticeable crunch if I strummed harder. So with only a minute of two of messing around, I found what I thought was a very expressive setting, allowing me to control not just volume, but overdrive as well, simply by varying pick attack. And I feel like I'm just scratching the surface of what this can do tonally.


mitch m wrote:
Here's a picture of the outside of the chassis.


Is that piece of metal between the big caps and the power tubes like a heat shield?


Hankules wrote:
In any case, sure is "priddy" (Okie for pretty).


+1 to that!


mitch m wrote:
I'm happy to answer your questions, but this is sort of hijacking Matt's post about his successful Triwatt build. I have lots more pictures of my DR504 project and I can post some of them in the Dressing Room section if anyone is interested in seeing what a Triwatt's big brother looks like.


No worries! I've been doing online discussion forums so long (starting back in the dialup BBS days, then Usenet), I'm no stranger to thread detours. Can't say I haven't caused a few myself!


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PostPosted: Sat Dec 07, 2019 9:00 pm 
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Sorry, Matt :oops: I just wasn't sure from Mitch's first pic and couldn't tell what I was seeing. Mitch, YES! Always looking for techniques :thumbsup:

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PostPosted: Sun Dec 08, 2019 5:49 pm 
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Looking at the schematics, a TC-15 is pretty close to a DC-30 running at half power. So it should sound pretty much like your Dizzy 30 with two of its output tubes shut off. I still have my TC-15 and I really like it, but I haven't played it too much lately. I also have a Trinity Tweed Deluxe that I play most of the time.

The metal piece between the output tubes and the filter capacitors on my 504 is a heat shield like the original Hiwatts had. A couple of EL34 tubes get fairly hot so it helps to keep the capacitors from getting too warm. Some Hiwatts have four EL34s so they would get even hotter.


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PostPosted: Sun Jan 05, 2020 6:29 pm 
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Hey Matt,

I am very glad to hear that my pictures helped you out.
Ultimately, that was the reason for me to document all the different steps with high-res pictures.

Enjoy your amp!


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PostPosted: Wed May 13, 2020 4:28 pm 
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Reviving this thread. I had my Triwatt at my buddy's house (rehearsal space) to give it a proper break-in. Rehearsals have been suspended due to coronavirus, and I've been without the Triwatt until this past weekend when I retrieved it from my friend's house.

I played on it a bit the other night to refresh my "aural memory", and I think I want to do a little tuning. But first, I want to tackle the issue with my bias voltages being a little low. Hopefully these choice quotes are enough to save re-reading the whole thread:

matt_garman wrote:
My 6V6 tubes are at about 15mV bias right now. This is actually with the trim pot maxed out: the range seems to be about 9.5mV to 15.5mV. The docs say 6V6 bias should be between 15 and 23 mV. So I think I should be able to go a little higher.


mitch m wrote:
But you should see about -54 volts at the junction of D6/R45/C22 and it should be steady.

At the junction of R45/R46/C23 the voltage should be closer to -45. It should vary with the setting of the bias pot...


matt_garman wrote:
OK, here's the measurements I'm getting with 6V6 tubes and 122 VAC mains.

Notation: my measured value (documented reference value)

At the D6/C22/R45 junction, I measure -58.8 (-54).

At the R45/C23/R46 junction, I measure -44.5 (-45).


mitch m wrote:
You could try changing the 180K resistor in the bias circuit. If you want to run KT66 tubes the bias will probably be low for them too.

As an experiment, if you have a 1Meg resistor you could clip or tack-solder it in parallel with the 180K and see what that does. That will reduce the 180K down to about 150K so you should have more bias range.


coco wrote:
For the bias, make sure your switch is set to 6V6 position (lower B+ & Bias voltage position incorporating the Zener diode and 180K bias resistor, and the correct bias resistor. If it is still low, you can make the voltage MORE negative by adjusting that resistor to say 150K. Just Remember that the KT66 position puts the two (180K//620K == 140K) in parallel to obtain a higher bias voltage.


So my main question is: clearly I can lower R43/180k as discussed. But is it worthwhile to trace "upstream" to see where the discrepancy is introduced? For example, the D6/C22/R45 junction is at -58.8V, where it should be around -54. Should I try to get that better matching the reference values (which would in turn likely correct my bias voltages)?

Basically, I'm just concerned that there is a bigger issue at play, and limiting the fix to only lowering R43/180k is just a band-aid. I don't want to mask some other part of the circuit that might be operating out of expected design parameters. On the other hand, perhaps this is just due to the cumulative sum of component tolerances? Dunno, that's why I'm asking. :)


For a little context: I feel the tone should be a little smoother---maybe the highs are a little on the raspy side? And I'm not sure if the "problem" is an issue with the circuit, or just my expectations. But either way, I feel the circuit should be confirmed correct before I start tweaking things. And that's the line of thought that leads me to the question about the bias (i.e. lowering R43 good enough, or should I try to dig deeper?).

Thanks!


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PostPosted: Thu May 14, 2020 1:31 am 
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It's most likely just normal component variation that's reducing your bias range rather than a problem upstream or downstream of R43.

I helped Mark (rotorcraft230) recently with a mod to increase the bias in his Triwatt so he could run EL34 output tubes. The thread is five down from this one if you want to read it. We found that to increase the bias range he had to increase the value of R43. He ended up using 220K.


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PostPosted: Thu May 14, 2020 2:49 pm 
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mitch m wrote:
It's most likely just normal component variation that's reducing your bias range rather than a problem upstream or downstream of R43.

I helped Mark (rotorcraft230) recently with a mod to increase the bias in his Triwatt so he could run EL34 output tubes. The thread is five down from this one if you want to read it. We found that to increase the bias range he had to increase the value of R43. He ended up using 220K.


Just to help anyone who reads this after the thread ordering changes, this is the thread to which Mitch is referring: Anybody using EL34 in their Triwatt?

I happen to have a few 680k and 620k 1W resistors. Two 680k and one 620k in parallel make 219k, so I'll give that a try. If that proves successful, I'll order a single 220k for neatness.

Thanks again for your tireless help!


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PostPosted: Fri May 15, 2020 10:42 am 
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OK, good news and bad news.

First the good news: turns out all this bias issue rigamarole was a classic "ID-10-T" error: I had the bias switch flipped the wrong way. See my post about bias switch clarification. I should have taken the time to actually understand the bias switch situation much earlier in the process, but.... that's the bad news, my pride is definitely damaged. :oops:

In other words, for all the prior discussion, the bias switch was in the KT66 position, and I'm using 6V6 tubes. I didn't make any changes to the amp, other than flipping the switch; and now, with the trimpot I can adjust the bias pretty much through the exact range suggested by the docs for 6v6 operation. These are approximately the ranges I see with 6V6 tubes:

KT66 setting: -9mV to -15mV
6V6 setting: -15mV to -23mV

Since I started out on the KT66 setting, I just assumed that those low mV readings meant I was actually on the 6V6 setting. I should have just flipped the switch, but I (wrongly) assumed that would make the bias go too high for the 6V6 tubes. (And due to my initial error of the missing wire in the bias circuit, I was already shy about more potential red-plating of my tubes.)

Where's the facepalm emoji?

As established the EL34 thread, these negative voltages are somewhat counter-intuitive: smaller bias resistor means less-negative bias voltage. The KT66 setting is a smaller resistance, since it adds another resistor in parallel.

If I'm thinking about this correctly (or maybe I'm about to further embarrass myself) I believe it explains Stephen's statement which originally confused me:

coco wrote:
Note: If the amp is fitted with KT66s and the switch moved to the 6V6 position, that could damage things, since KT66s require a higher bias voltage. With 6V6s, there is no danger with the switch in either position, although the amp won't sound its best, since the internal voltages will be a little too high.


As shown above, with 6V6 tubes, with the switch in the wrong position, you get a bias that's too low (smaller effective bias resistor). So presumably, with KT66 tubes installed, on the 6V6 setting, you'd have a bigger effective bias resistor, which means the bias setting would be way too high. In fact, if I go back and review the Triwatt documentation that explains biasing, I could probably do the math to predict the bias values with KT66 tubes on the 6V6 switch position.

As a side note, for further confirmation of my folly: I do in fact see a lower B+ voltage with the switch in the 6V6 position (as expected). Could very well be expectation bias, but I do believe the amp sounds better now!

So now my worries of there being some bigger upstream problem are completely unfounded, so I have much more confidence that the circuit is performing as intended. Now the guy operating the circuit... well.... hopefully future builders can learn something from this discussion!


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PostPosted: Sat May 16, 2020 1:09 am 
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Thanks for the clarification, Matt. Don't worry, you aren't the first one to get confused due to incorrect switch settings!

So these are the readings you get at the bias test points?
KT66 setting: -9mV to -15mV
6V6 setting: -15mV to -23mV

If so, that means 6V6 gets higher bias current than KT66 and Stephen's comment makes sense. For some reason I thought it was the other way around, too.


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PostPosted: Sat May 16, 2020 3:05 pm 
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mitch m wrote:
So these are the readings you get at the bias test points?
KT66 setting: -9mV to -15mV
6V6 setting: -15mV to -23mV


Yup, I'm not 100% sure about the smaller number on the KT66 setting, as I'm going from memory. But definitely -15mV is the one end, because that's where I had it parked all this time I mistakenly thought it was on the 6V6 setting.


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