One More Tweedle Dee Build

[Blue Parrot]

Martin, yes the documentation above does not inspire much confidence.

Hi Plexi - Seems that the 330-0-330 transformers work well for this amp. I considered grounding my HT CT through a zener to drop the high voltage. However, the solution I actually pursued was to build a simple bucking transformer to drop my line voltage from 123 VAC to 115 VAC. From measurements using a variac (see above), I know 115 VAC results in a heater voltage of 6.12 VAC and a B+ of 398 VDC using a GZ34S rectifier tube. I think that as long as the filament voltage is above 6.00 VAC that this should work. Photos of the bucking transformer are shown below.

I believe the bucking transformer will work for me, at least for the time being. If it doesn’t, I may revisit using a 330-0-330 PT, and will certainly consider the Mojo transformer. Thanks Plexi for the PT suggestion.

I have some final parts on the way and once I find time to assemble, I will post photos of the completed amp.

[martin manning]

Nice box. Filament voltage is fine at 6.3 +/- 10%, so as low as 5.7. Another option: Magnetic components 40-18078 for 5E3 has dual secondaries, 330-0-330 and 355-0-355 @100mA, with 100-110-120V primary.

[Blue Parrot]

I wanted to document the final stages of construction. To verify the bucking transformer operates properly, I plugged into the 115 VAC outlet and measured voltages in the amp using the GZ34S rectifier tube. These voltages are posted below and agree closely with Charlie’s original observations. The B+ was 400 VDC and the heaters measured on V1 were 6.16 VAC. Plate dissipation was calculated at 16.7 watts.

I installed the completed the amplifier and a Celestion Rocket 50 speaker into a cabinet constructed by John Mergili. Photos are included below. Total weight of the combo amp including the bucking transformer was a very portable 28 lbs.

I did some measurements looking at the operating temperature of the amp that I will post later today.

[Blue Parrot]

A number of builders have expressed concern that this amplifier gets hot. I also noted my build seemed to give off a lot of heat while doing the initial voltage measurements. Some have accepted this and others plan to facilitate cooling by including a fan in the cabinet.

I wanted to do some temperature measurements to make sure the amp wasn’t getting too hot for the components. These measurements were performed on the completed combo amp (chassis installed in the cabinet with the cabinet back fastened). To do this I inserted a thermocouple into the chassis through one of the inner input jacks and securely fastened so measurements were taken from inside the chassis about 1.25 in behind the volume pot nearest the inside input jack. The amp was turned on and measurements were taken periodically until thermal equilibrium was reached. At that point the amp was turned off and the cooling curve recorded. These data are shown plotted below. The data points in green are when the amp is on, whereas the readings shown in red are after the amp was turned off and cooling down. Ambient temperature was 23.8°C (75°F).

I think this study presents some useful general information, although the exact curve would be dependent on a number of variables including probe position, chassis material and dimensions, and the cabinet used. An equilibrium temperature of 55°C (131°F) was reached in this amplifier after 113 min of operation. This was lower than I was expecting and was a very pleasant surprise. So in this specific amplifier, I don’t think I will need to worry about overheating problems.

I am reluctant to post impressions of the amp with only 6 hrs of playing time because past experience tells me not to rush to judgment as the amp may take a while to break in and I might need to try some different tubes in V1and V2 (I think I am restricted to JJ’s for the power tubes). That said, I find this amp to sound very nice for the type of blues I play, so this amp will definitely get used. My only concern is the amp is slightly fatiguing for me to listen to for long periods. I suspect this will improve as the amp and speaker log some use.

I again wanted to thank Charlie and the TAG members that have tackled this build before me. Thanks also to members that took the time to comment and help me work through issues brought up in this thread.

Scott