isolation transformer won't supply filiments - 1968 Harmony H400A tube guitar amp

[tonebrulee]

Hi,

I am pretty new to electronics and am working a small tube amp project. The amp is a Harmony H400A (tube complement: 5OC5, 12AU6,35W4) from 1968. These amps have a non-polarized two prong chord and no power tranformer, making them both pretty unsafe and subject to lots of hum. I have added a grounded 3-prong plug to address safety, but would also like to add an isolation transformer.

Not having access to a small iso-transformer and doing some quick research, I picked up two $6 120VAC-to-25.2VAC 450mA transformers at RS and wired them back to back. Introducing this into the hot/neutral circuit before the amp, the amp works (less hum!), but the volume is significantly reduced, AND I can see that the tube filiments only glowing dimly relative to when not using the iso-tranny. With the amp turned off, the output on the amp-side of the iso-tranny is 112VAC (down 8V from the initial 120VAC reading from the wall). More significantly, when the amp is turned on the same reading drops to 75VAC (without the iso-tranny, the reading at the hot/neutral connection to the amp remains 120VAC regardless of whether the amp is on or off). Any insight into what is going on here?

I then found info suggesting that the hot lead from the wall could be wired directly to the tube filiments, bypassing the iso-tranny. I tried this, but the filiments then received NO power at all. Is there something I am missing here trying to wire the filiments to the wall? Perhaps the filiment neutral needs to go directly to the wall too?

Thanks.

 

[PRNDL]

Most likely the Radio Shack transformers can't handle the load.

You will be much better off getting a Triad 553-N68X isolation transformer from Mouser. It's less than $20.

Most likely you connected the filaments to AC neutral (white) instead of hot (black), however, this is VERY dangerous!!!!!

Perhaps the fiiment neutral needs to go directly to the wall too?

That might be the problem, since you've isolated the return.

 

[Tom Bavis]

Those Radio Shack transformers are only about 10 VA (volts x amps) and your amp needs 18 VA for the heaters alone... plus maybe another 10 VA for the B+. A 50VA isolation transformer (as suggested above) would be good.

 

[tonebrulee]

Most likely the Radio Shack transformers can't handle the load.

Those Radio Shack transformers are only about 10 VA (volts x amps) and your amp needs 18 VA for the heaters alone

Thanks much. I ordered your suggested XFMR from Mouser, but I'm curious about the "10 VA" you mentioned for the RS transformers. Is this based on the secondary output (25.2VAC) x 450mA = 11.3VA ?

 

[Eli Duttman]

Just using the 50 VA Triad N-68X is not going to solve the problem, sorry. The 35W4 is a half wave rectifier. Half wave rectification puts a "standing" DC voltage on the power trafo. The N-68X can't tolerate that DC.

IMO, the least painful solution to the 1/2 wave problem is bridge rectifying the secondary of a N-68X with 4X UF4007 SS diodes and connecting the bridge's O/P to the 35W4's anode. Good practice requires the diodes on the negative side of the bridge be paralleled by 1000 WVDC/10 nF. ceramic capacitors. The ceramic caps. keep SS diode switching noise out of the power trafo. The 35W4 keeps the "hash" out of the PSU filter.

Modernizing old, unsafe, transformerless, AC/DC equipment has its challenges.

BTW, today's higher average line voltage may require the 150 mA. series heater string dropping resistor be replaced. A 10 W. rated 150 Ohm part is indicated.

 

[tonebrulee]

half wave rectifier

Thanks Eli. The original circuit has the non-transformed AC+ mains going straight into the rectifier. I am very new to this craft: can you explain why the transformed supply would need to be treated differently? I'm guessing this has to do with some difference between current coming out of the wall vs. a power transformer that I'm not understanding?

Thanks again.

 

[Eli Duttman]

Dude,

Look at the OEM schematic. Notice that the negative side of the CRC B+ filter connects directly to the neutral wire in the wall outlet. Inserting a power trafo between the wall and the amp causes the negative DC voltage to be applied to the trafo's secondary. Transformers are AC devices that run into magnetic saturation from DC. Yes, the OEM circuit causes DC to be applied to the power company's distribution channel trafo, but that thing has a huge core and the flux due to the DC is small in proportion.

Installing the SS full wave bridge rectifier in front of the 35W4 removes the unbalanced DC from the isolation trafo and prevents core saturation.

That schematic surprised me. I was expecting a transformerless setup, as found in AC/DC AA5 radios. That circuit can't work on a 110 VDC power grid, because of T2. T2 supplies power to the 12AU6 heater and allows that heater to be grounded. The design pays some attention to safety. The chassis is not directly connected to the power mains. C6 and R3 are barricades, of a sort. However, if either part shorts out, a deadly dangerous connection to the power mains is created. C6 should be replaced by a 630 WVDC/0.033 muF. cap., Panasonic part # ECQ-P6333JU. R3 should be checked for physical integrity and value drift.

Add a 5 W. rated 10 Ohm resistor in series with R10 as an accomodation to today's higher average line voltage.

 

[tonebrulee]

Thanks Eli for illuminating how the electric company trafo is, in fact, a part of the circuit. As a newcomer, schematics make it easy to miss this by stopping at the plug.

I will definitely take your advice and replace C6. R3 is right on the money - after 40+ years!

I'm curious about adding the SS bridge rectifier - could this cause the circuit to be oversupplied? If not, how do you think it would effect the amp's performance in practical terms?

Lastly - might boosting R10 drop heater output, causing tube output to drop?