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Just completed the Trinity 15 build from the schematic on your website. Nice amp. I built it by modifying a completed Spitfire turret board and using old tranny's, a Baldwin organ PT and an older Fender HRD OT. Three main questions: 1) Is the HRD OT (HRD uses two 6L6's and much higher B+) hurting the EL84's in any way, and how will it "influence" the tone produced? Should I immediately replace with a proper sized, smaller xfmr?; 2) My plate voltage is 367 and screen voltage is 280, at idle (preamp gain and master volume both at 0). Does this make sense? Is this ok, plate and screen voltages being 87VDC apart? In an ideal build, what should the difference be? I replaced R25, the PS 22K screen grid resistor, with a 13K because the differential was even larger, 125V, with the 22K.; 3) with both gain and volume dimed, I get an extremely low hum (no problem, as far as I'm concerned), but I do get a nice consistent hiss (not overwhelming, for sure), which begins at about noon on the gain control with the volume dimed. Is the hiss possibly coming from V1? Is one brand of tube better than another with regards to minimizing hiss? BTW, the hiss never comes into play as I never run the gain beyond 11 o'clock anyway, too much distortion. Just looking for a V1 quiet tube suggestion. Or maybe it's not coming from V1?

Your OT will not work. The primary is probably around 4K.
The TC-15 OT is 5K-8K.

Also your EL84 are probably running hot at that B+. I'd check your cathode current and check plate dissipation. You must be using an interesting power supply to get the screens so much lower.

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kurtlives, thx for the info, very much appreciated.

The Trinity 15/Rev. 10 (dated 10/6/05) schematic that I have from the website says 4K Primary for the OT. I guess my question should have been "4K what?" The HRD OT says primary impedance 4.25K ohms CT... Where am I confused? An acquaintance who builds amps said that the OT will be fine. I didn't believe him, but I built it this way anyway. He gave me the trannys...

My PS build matched the schematic exactly, except for the Baldwin PT and the HRD choke (choke drops ~2 volts). I used six electrolytic caps (two 33uf/450V and four 22uf/450V) and four power resistors (22K, 3W), as per the schematic. Today, my PS voltages at idle: V0=369, V1=281, V2=336, V3=278, V4=348. The schematic shows: V0=352, and V1 thru V4 all at 329. What am I missing? The screen grid voltage was 246(!) before I decreased the screen resistor to 13K.

The PT high V sec is supplying 295VAC; schematic says 285.

What am I missing?

That schem you are looking at is old. Look at this viewtopic.php?f=12&t=859

Use the 1K5 resistor instead of the choke for now.

Wire the PS in the parallel arrangement like the scheme shows. Sounds like your wiring may be off.


EL84's here like about 8K primary impedance. The Crunch/Munch switch allows them to run at 5K or 8K. They are better suited to 8K though.

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kurtlives, Again, thx for the assistance. I'll pour over this updated schematic tonight!

You can run at 4K (Matchelss does in their DC30 at half power for example) and get a crunchy tone.
We do 5K & 8K for our crunch & munch. 8K is ideal.
f you used Carbon comp resistors, it's probably from them, you could change some of them if you wanted, or try a Tung Sol 12AXy preamp tube in V1.

Yes, check your power supply againse the latest schematic for the TC15.

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

coco, thx for the information. I re-checked the PS wiring against the updated TC-15 schematic referred to by kurtlives, and all looks correct. Exceptions are that I'm using 22uf vs 20uf electrolytics, a Fender 022707 choke (4 Hy/50mA from an HRD) vs the 1k5 resistor, and no standby switch. My electro's are Sprague's & Nichicon's. My screen grid resistor is now 13K in order to elevate the screen grid voltage.

I re-checked voltages thruout the amp, matching your response of Oct 5, 2011 (the last msg on the thread referred by kurtlives, the schematic revisions thread).
This is what I have (yours first/mine second, and I used a Fluke):

B+ - 380/368; AC input - 119/120
V1: 1&6 - 127/139; 3&8 - 1.46/1.40
V2: 1 - 170/174; 3 - 1.5/1.4; 6 - 274/276; 7 - 170/174; 8 - 171/174
V3: 1 - 257/264; 2 - 48/47; 3 - 70/69; 6 - 252/257; 7 - 47/46; 8 - 70/69
V4&V5: screens - 232/278; plates - 376/363; cathode - 8.1/9.1

As you can see, my voltages are close to yours, except for the EL-84's screen voltage. I reduced the screen grid resistance from 22k to 13k. I like higher screen voltages (but less than 300), and would like to see a lower plate voltage. My targets are 280VDC for the screens (close enough now) and 325 on the plates. Am I making a mistake doing this? Will this drastically effect tone? Are my other readings in an acceptable range?

I'm going to order a new OT (Edcor XPP15-8-8K, $21; I like Edcor & their pricing). Will this mod, by itself, provide additional clean headroom?

I'm a 61 yo rookie at guitar amps, but I've been "hobby soldering" (amateur radio, hifi) for half a century! I tend to ask many questions, sorry...

Thx again for your assistance on this project. Pete

The resistors going to pin 9 of the EL84 should be 100 ohms? Are they 100 ohms or something much larger?
The other end of these resistor(s) should be going to the screen supply node. In your case this will be one end of the choke. The other end of the choke will be going to the B+.

Edcor makes nice iron. Should be an improvement over what you have now. But no it won't help with headroom, if you want headroom then you have to make changes to the circuit.

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With an 8K , you will probably get a smoother, louder sound, with less grit/crunch. If possible, overspec it to handle 25 + watts.

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

Again, thx kurtlives and coco.

Yes, I did use 100 ohm (1%/3W, Vishay/Dale CPF3 metal film) right at the EL84 #9 pins. My question back is why are these needed if a 22k resistor (or 13k in my case) is directly connected to these, accomplishing the voltage drop?

I don't read the schematic the way that you are. The TC-15 schematic shows the two 100 ohm screen resistors connecting to V1, which is the first RC filter, 22k/3W resistor & 22uf electrolytic, and not at V0 at the choke. Am I missing something?

What you are describing is a Spitfire-type circuit. On the Spifire, the plates and screens are basically at the same high voltage, around 350VDC. I tried this initially, by the way, because I had the Spitfire turretboard already constructed. The screens appeared to be glowing and not what I wanted, so I added a 22uf/22k RC filter to drop the screen voltage, like the Trinity 15.

I've read that I could have gone directly from the 22k screen grid resistor right to the two tubes. So why the two 100 ohm resistors? Or did I get this wrong?

I'm definitely replacing the Fender OT with a 25W Edcor.

Thx again guys. I appreciate your comments immensely. Pete

The two screen grids are running anti-phase to each other, so unless there is some resistance in the path between them, they will will push current into each other on alternate half-cycles. Having the 100 ohm resistors helps limit that. If the screen grids still glow, then you can try 1k instead.

Around 330V should hit the sweet spot for anode (aka "plate") voltage.

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zaphod, thx for the explanation.