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PostPosted: Thu Jul 02, 2020 1:18 pm 
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17 months after receiving the kit, I finally got around to completing the build. Following the manual to a "T", the only issue I had was this god-awful squawking on power-up with tubes, which was quickly remedied by swapping OT primaries.

So far things are sounding really good. Amp is dead quiet even when turned up. I'm running a pair of Gold Lion reissue KT66, NOS military Mullard 12AT7 as the PI, and mix of current production 12AX7s in V1-3. The power tubes are biased at 35mA.

While most of my voltages look pretty good, my B+ is lower than I expected at 410VDC. My HV on the secondaries was about 650VAC. (all values with tubes).

Power tube voltages (same for both; KT66)
Pin 3: 410
Pin 4: 384
Pin 5: (-)45

I don't want to get bogged down with numbers if the amp is sounding great, but just wanted to see if there is any reason for concern with the voltages. For what its worth, voltage readings on V1-V4 are all within 3-15% of the stated readings on the voltage table on page 52 in the manual (albeit the table was derived with 6V6 tubes installed).

Anyways...very happy with the build...will try some pedals in front to see how it responds. The OD circuit really bumps up the volume but gives a tasty classic crunch. Even with the OD engaged, the amp is pretty quiet.


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PostPosted: Thu Jul 02, 2020 9:48 pm 
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Is the issue volume or voltage? Per Trinity website, specs on the the Triwatt indicate 33W using the KT66's versus 22W with 6V6's. :) That should be pretty darn loud. :bugeye: PS The Voltage Chart on pg. 52 shows voltages for the 6V6, might (should??) be different for the KT 66's, just a thought.

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PostPosted: Fri Jul 03, 2020 12:44 am 
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The voltages could be lower with KT66 tubes. Maybe somebody who is running KT66s in a Triwatt can check the voltage at pins 3 and 4 to verify this.

You do have the 6V6/KT66 switch in the KT66 position, right? In the KT66 position you should read 0 ohms across D5 with the power off. If it's wired correctly, the switch bypasses D5 in the KT66 position, increasing the B+.

What does the incoming AC line voltage read? Maybe that's low. Also what does the voltage on the heaters read?


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PostPosted: Fri Jul 03, 2020 2:36 pm 
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Thanks for the suggestions/feedback! I definitely have the switch oriented correctly...I totally obsessed over that part since it was a little confusing based on the layout and manual. This forced me to really hone my schematic reading skills. Basically, KT66 is when the "wired" end of the switch is engaged (bat of switch toward the OT) and the 6V6 setting is when the "NC" end of the switch is engaged (bat of the switch is towards the PT).

As far as the volume...heck yeah those KT66's are LOUD!!! But gawd does it sound good cranked up! 8)

I am going to do a complete table of KT66 and 6V6 readings and post them here later this afternoon. There's really nothing wrong with the amp...I was just a little concered regarding the B+ being a little low (see my post last April 2019 re: low voltages on initial PT hookup).


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PostPosted: Fri Jul 03, 2020 4:18 pm 
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OK so I took measurements with KT66 (Gold Lion reissues) and 6V6 (RCA Blackplates).
First just let me say this amp with the 6V6 tubes, OD engaged and master around 2 o'clock sounded amazing (will try to make some soundclips when I get a chance).

With the numbers below, the K refers to KT66 installed, V refers to 6V6 installed, and the parentheses refers to the numbers in the table on p52 of the manual (or layout diagram as noted). All readings taken after 10min of idle. Readings taken within about 30min of swapping tubes. Overall the numbers look good except for low HVAC and B+ and V5/6 Pin 3 and 4. V1-4 are almost spot-on. The only other value that sticks out is V4 Pin 6. Amp is sounding good, all controls operational and its pretty quiet.


Wall voltage (VAC): K117.1, V117.9 (122)
Heaters(VAC): K6.18, V6.33 (on standby)
HVAC: K665.4, V686.1 (no reference)
B+: K430, V427.4 (442)...note: for 6V6 setting, B+ was 432.2VDC prior to Zener
Bias: K = 34.7 and 34.1mA; V = 18.4 and 16.4mA

V5 and V6 (values within 1 VDC on each tube, so reported just once)
Pin 3: K428.6, V426.5 (441 per layout)
Pin 4: K 402.2, V400.0 (411 per layout)
Pin 5: K-48.2, V-39.7 (-39 per layout)

V1 (Mullard Reissue 12AX7)
Pin 1: K150.4, V146.7 (150)
Pin 2: K0, V0 (0)
Pin 3: K1.1, V1.1 (1.2)
Pin 6: K133.1, V129 (120)
Pin 7: K0, V0 (0)
Pin 8: K0.9, V0.9 (1)

V2 (JJ 12AX7LPS)
Pin 1: K182.5, V178.2 (179)
Pin 2: K0, V0 (0)
Pin 3: K1.1, V1.1 (1.2)
Pin 6: K106.2, V102 (106)
Pin 7: K0, V0 (0)
Pin 8: K0.7, V0.7 (0.7)

V3 (Mullard Reissue 12AX7)
Pin 1: K264.3, V260.7 (269)
Pin 2: K65.7, V64.4 (67)
Pin 3: K69.3, V68.3 (71)
Pin 6: K183.7, V180.8 (188)
Pin 7: K0, V0 (0)
Pin 8: K1.5, V1.5 (1.5)

V4 (NOS Mullard military 12AT7)
Pin 1: K267, V266 (274)
Pin 2: K69.4, V68.4 (71)
Pin 3: K72.7, V71.7 (74)
Pin 6: K262, V263 (236)
Pin 7: K63.3, V62.4 (64.4)
Pin 8: K72.7, V71.7 (74)


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PostPosted: Sat Jul 04, 2020 12:58 pm 
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Good to document all those voltage readings. There does appear to be some differences using KT66 tubes, but that's to be expected. The voltages in the table are average ones and some variation is expected there as well. All your voltages are within the usual tolerances so nothing stands out as a problem.

If the amp sounds good it is good!

The wall voltage is a bit low which affects the B+ and the heater voltage, of course. A couple of volts down at the primary will make the B+ about 20 volts low. Again it's nothing to worry about. Line voltage varies from place to place and throughout the day.


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PostPosted: Mon Jul 06, 2020 4:28 pm 
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Thanks for the feedback Mitch...yeah my wall voltage fluctuates throughout the day at least 5 or 6 Volts, so that's likely the culprit. I guess everyone was draining the grid with the AC on full blast during this 95 degree heat in the afternoon down here.

What do you think about my before/after Zener readings? I thought the Zener was supposed to drop 18 volts when the 6v6 switch is engaged....but I measured only a 5 volt drop. In the grand scheme of things I think its ok since my overall B+ was a little lower anyways, but just curious.


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PostPosted: Mon Jul 06, 2020 6:06 pm 
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Yes, according to the documentation the Zener is supposed to drop the B+ by 18 volts. I don't have a Triwatt to verify this, though. Maybe somebody who has a Triwatt can measure the B+ in both switch settings to see what the difference is.

Check the Zener to see if it's a 1N5355B as the schematic says. The number should be stamped on its body. A 1N5355B is a 5-watt 18-volt Zener diode. If it says something else, look up the number on the Internet and see what the specs are.


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PostPosted: Wed Jul 08, 2020 11:22 pm 
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Hi Mitch,
It is in fact the correct zener...I actually took a close up pic during the build to make sure I had the correct part number.

When I get a chance I will re-do some measurements in both switch positions. I would guess the best place to measure is either right befire R40 or on the preamp side of the Zener, correct? Amp would be in play mode.


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PostPosted: Thu Jul 09, 2020 1:29 am 
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Yes. You can measure the B+ at the end of R40 closest to the pots. That's also where the center tap for the output transformer and the wire to the standby switch should connect. You could also measure it at the + terminals of C20, the anode of the Zener, or the standby switch. The standby switch should be closed (the amp is running).

Sounds like you have checked all your wiring for the power supply and to the KT66/6V6 switch numerous times and it's all OK. The wiring in this area looks pretty busy in the layout drawing. I can see how it would be easy to get something a bit wrong. Also I assume you have the Zener oriented properly with the band end connecting to the band end of the power supply diodes.


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