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Hi: My 18W sIII V6 kit arrived the other day. I am going to build my sIII gradually over several months. I previously built a Tramp (back in 2012) and I know that it is best to take things slow and steady and to double-check everything. I also know that it is helpful (if not essential) to get input and advice from you veteran builders on this forum.

I already had to ask Stephen by email about the new shorter turret board he is now using for the V6, which has the Cathode Capacitors stacked “piggybacked” on top of the Cathode Resistors; hence the name “piggy back” layout, and the name I am using for this thread/topic.

I do have two preliminary build questions as I begin my slow and steady kit build.

First, the Output Transformer.-- The manual says to tie off the 5k blue/white and brown/white primary wires and put then “outside the chassis”. However, I noticed that the “18 Watt Plexi Pictorial Build” on the Resource section of the forum shows these two wires tied them off with shrink tube but puts them inside the chassis (under the turret board).

Question: does it matter if the two tied off wires are put inside the chassis (under the turret board), or is it better to put them outside the chassis?

Second, the Power Transformer-- The manual says to orient the PT so that the (green) “6.3 VAC heater wires are facing the rectifier” which means that the red side (580v side) will be facing the power switch and standby switch. However, I noticed that the “18 Watt Plexi Pictorial Build” shows the PT oriented in the opposite direction (with the green 6.3 VAC side facing the rectifier).

Question: which is the preferred orientation? Is there a reason for the preferred orientation?


That's all for now. Hopefully I will post pictures and show progress over the next few months, and get suggestions and help from you 18watt veterans along the way.

Thanks in advance for your needed advice

Cecelius2

It's good to take your time with a new build and ask lots of questions if you have to. I don't think I could wait several months to try a new amp once I start building it, though!

Maybe it has been found that the amp has less hum with the power transformer oriented with the heater wires facing the rectifier instead of the way it's shown in the “18 Watt Plexi Pictorial Build." I would build it the way it's shown in the pictorial, though. The wire runs are shorter and more direct.

As for the output transformer, I would think it's safer and neater to keep the unused wires inside the chassis. I would also be interested in knowing the reasoning for leaving them outside.

Short, direct wires are good but we found it doesn't actually make a lot of difference to noise..

We leave the unused OT wires outside because they can interfere electrically and make noise.

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

Question about continuity between heater wiring after installation of Power Transformer.

I just installed the long twisted red and black heater wires to all of the preamp tubes and power tubes. At each tube, I checked for continuity between the red and black wires at each tube, and confirmed that there was NO continuity, (no short) between the red and black wires; again I check at each tube as the heater wires were connected. There was no short, no continuity, which is good.

Next, I then installed the Power Transformer and connected its two GREEN wires (2x 3.15v =6.3v ) to the red and black heater wires (at the the larger terminal strip); I then double-checked for any continuity back between red and black wires again at each tube; however, now it shows some continuity. Further, when I hooked up the Power Transformer's ground wires (green-white and red-yellow) to the power ground star, I found that the red and black heater wires now also show continuity with the ground. FYI—I used a continuity tester/probe which lights up when there is continuity (see attached picture).

I suspect that what I now see as continuity is showing up because the Power Transformer is now in the circuit and this is normal. Is this CORRECT?

I do not want to proceed with the next step to “Test the Power Supply” (as the Manual instructs) until I hear back from one of you experts here that this continuity is showing because the PT is now in the circuit and this is normal. I don't want to fry the PT if this is not normal.

So, can one of you experienced gurus confirm that what I see is normal?

Thanks

Yes, this is normal. You are just reading the power transformer with your continuity tester.

A transformer winding has a really low DC resistance, especially if it's a low voltage one. If you used an ohm meter you would see that it's really about .4 ohms or so, not a short. And it would be half of that from one end of the winding to ground, since the winding's center tap is connected to ground.

Go ahead and do the voltage checks. You should see about 6.3 volts AC across the heater pins at each tube socket and about 3.15 volts AC from each heater pin to ground. The readings will be a bit higher with no tubes providing a load. It depends on the line voltage coming in, too.

You can also test the high voltage, but be very careful!

Mitch: Thanks very much. This is what I suspected, but I wanted to be certain.

Thanks

I just did the first "Test the Power Supply" check to see if the Power Transformer is sending the appropriate voltages to the heaters. This is with no rectifer tube (and no other tubes). Here are my readings:

Mains Voltage = 116.5vac (fluctuates up to 117.2)
Rectifier (pins 4 & 5) = 6.5vac
Rectifier (pins 1 & 7) = 640vac (fluctuates up to 660)
6V6 pins (pins 2 & 7) = 6.5vac
EL84 pins (pins 4 & 5) = 6.5vac
V1, V2, V3 (pins 4/5 & 9) = 6.5vac

My only questions is about the Rectifier 640v across pins 1&7. Is the 640v okay? It is noticeably higher than the 580v target.

Thanks for the input, help and advice.

C2

Looks like everything is working. Voltages will read high with no load on the transformer. Do you get about 6.25 VAC from each heater pin on the preamp and power amp tubes to ground?

Mitch: Didn't you intended to say "3.15vac from each heater pin to the ground"? That is what you wrote in you previous post a couple days ago.

So, I just checked mine again to see what the readings are to ground. On each of my preamp and power amp tubes I get 3.1x vac to ground. Unfortunately my multimeter only reads down to one decimal point below "0".

It looks good to me. Please just confirm that you meant 3.15vac to ground.

Thanks

C2

Yes, I meant to type 3.25 which is half of the 6.5 you measured across the heater pins. 3.1x is close enough.

Two Questions about the “Tube Cathode Switch” (v6):

First, the DPDT switch has two 220k resistors, one on each side. When engaged, the switch shorts the 220k resistor. Question: how do I know which side is engaged? Is the side that is engaged the one where the switch shorts the resistor (removing it from the circuit), or is it the side where the resistor is “in the circuit”?

Second, can the v6 version be run with just the 6V6 tubes and no EL84 tubes, (or just EL84 tubes and no 6V6 tubes)?


Thanks,

C2

Usually a double throw switch will "engage" something in both positions, but different circuits will be engaged depending on what position the switch is in.

In this case, the switch is configured so it shorts the 220K resistor in series with the 6V6 cathodes in one position and the 220K resistor in series with the EL84 cathodes in the other position. This has the effect of "engaging" either the 6V6 tubes or the EL84 tubes. When the resistor is shorted, the tubes connected to it are active.

Physically, double throw toggle switches have terminals at the top and bottom and a common terminal in the middle. On most switches the terminals that are connected are the common one and the one opposite to the position of the toggle. You can verify this by checking for continuity with an ohm meter.

This is a double pole switch so you have two independent sets of contacts, one set on the left side and the other set on the right side.

You should be able to run the amp with just the 6V6 tubes and no EL84's or vice versa. As long as the switch is in the proper position.

I used 18 gauge solid wire [rated at 600v]from the rectifier pin 3 to the standby switch, but I just saw in the FAQ section of the manual that I should have used 20 gauge stranded wire for the rectifier to Standby wire. I am still building the kit and can go back and change it to stranded.

I also re-used the cutoffs from the output transformer for going to Standby to the B+ on the large can capactor.

Is solid 18 gauge not acceptable to use for the wire from rectifier pin 3 to the standby switch?

C2

You are good to go. Plenty of size there.

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

VRM Coupling and Cascade Mod option?

I am about to wire the V1 section; I will be adding the two extra VRM DC Coupling capacitors to pin 2 and pin 7, and will move the 1 meg resistors off the high inputs and put them at V1 as directed by the VRM manual [see first attached diagram below]. However, I am also adding the Cascade Mod (cascading the TMB channel into the Normal Channel); see the second attached diagram below for the Cascade Mod [but I will change it so that the TMB goes into the Normal channel].

Before I install the VRM 1 Meg "Grid Load" resistor from V1 pin 7 onto the ground tag onto the Normal Channel's the terminal strip, I want to confirm that the VRM's 1 Meg Grid Load resistor is still needed given the Cascade mod (TMB into Normal Channel). I suspect that it is still needed, but I wanted to check with you guys to make sure that having that 1 Meg resistor going to ground and having that Cascade mod will not cause a problem.

So are the VRM 1 Meg Grid Load Resistors going from pin 2 and pin 7 to ground both still used with the Cascade Mod?

Thanks again for all the very helpful advice.

C2

Yes, the 1M grid resistor is still required.

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

I wanted to post some pictures of my gradual progress. I should have posted these earlier, but I am just now getting to posting the pictures.

The first picture is of the hardware installed on the chassis. I made a few changes/upgrades. First I added an 8-tag terminal strip. The reason for this is that I wanted to have two extra tags available down the line for when I add footswitch relays (which will require an extra capacitor to go on these extra tags). Next, I also changed the 3-tag terminals around V1 to 4-tag terminals (these are for extra convenience when I eventually wire the two VRM DC Coupling Capacitors, which I will install on the fourth tags). The Third, Fourth and Fifth changes are the push-pull 500k potentiomenters for the Bass, Gain and Volume pots on the TMB channel. I will use the Bass push/pull pot for the cathode switch; I will use the Gain push/pull pot for the gain Boost switch; and finally, I will use the Volume push/pull pot for the Cascade Mod (cascading the TMB into the Normal Channel). These three push/pull switches will be more convenient for me to have on the front panel.


The second picture is of the 8 tag terminal strip (which gives six non-grounded tags)


The third picture is of the two 4-tag terminal strips around V1. I also put some tape in the V1 corner as some insolation protection.


More pictures to come.

C2

Here are some more pics:

First, Power Transformer and Output Transformer installed (OT primaries twisted and OT secondaries braided). The two extra 5k primary (blue-white, brown-white) tied off with heat-shrink and put outside the chassis (tucked under the OT).




Second, Power Transformer wired to mains, to switch with VRM, to terminal strip and to rectifier.


Third, closeup of wiring of the fuse and the mains (120v).


Fourth, VRM pre-wired prior to installation.



Fifth, I added a permanent 220k 3watt Drain/Bleed Resistor as suggested as an option in the manual. I installed the resistor across positive B+ side of the Canister Capacitor and connected it to power ground star. [See this thread for more info: https://trinityamps.com/phpbb/viewtopic.php?f=7&t=3415 ]


Next, I will post some pics of the turret board components

C2

Looks good!

Posting some more pics of progress

Heaters wired. I made a mistake and ran out of the provided red and black wires; so I added some 18 gauge cloth wire for V1 heaters.


Jumper wires installed underneath Turret Board; I added some yellow shrink-tube over the wires just extra insulation.


5 watt bias resistors placed on Popsicle sticks for an 1/8" gap.


Piggyback bypass capacitors stacked on top of bias resistors (again using Popsicle stick for spacing).


Popsicle sticks removed.


Also, I tinned (added solder to) the leads on both bypass capacitors to give them more rigidity and stability. I noticed that it was too easy to bump them and have them fall down too close to the resistors. Tinning the leads gave a better "posts" for their support.

C2