paraglow 45s

[rockdoc]

I have spare chokes, power (PGP 8.1)and output transformers (EXO-45-cobalt), and C4S kits I bought when I first built my paraglow 45 amps in 2001. Time to build from scratch using these parts. I want to put the hum trim pots up top on the plate, and maybe increase dimensions a bit. Any recommendations? Are appropriate top plates available to buy? And, if I were to go with a stereo amp as opposed to monos, has anyone done this with the paraglows and any advice? Thanks!

Steve T

[Paul Birkeland]

There are kind of two ways to go:

1.  Get a Stereomour, build it for 45's, substitute your plate chokes and OT's for what comes in the kit.  The operating point may be a little off for 5K, but pretty dang close out of the gate.  If you do go this route, you can simply resell your stock iron on eBay.

2.  Go all the way from scratch.  The PGP 8.1 has too much primary voltage to be useful for a 45 amp, so perhaps you can use a choke input filter or a small first cap to shed some voltage.  The other option is to go directly coupled like the Paraglow, but adjust the parts values for 45's.  To get top plates, you can download Front Panel Designer and order up what you'd want.  I suspect a pair of anodized plates will run you $100-150.

The Paraglow chassis plates and Paramount chassis plates are NLA, but perhaps there is someone out there who bought a kit and never ended up using the plates we included.

-PB

[rockdoc]

Thanks Paul!

When I built the Paraglows in 2001, the only change in the instructions was to replace the 2.5Kohm or 3Kohm 10 watt wire wound resistor for the KR 2A3 or the chinese 2A3; subbing in a 7K instead, which I did. Are there other changes I should make? And, I built the amps with the supplied PGP 8.1 transformers. Should I not use these trannys? Or , should I be dropping the primary voltage as you suggested? They seem to have worked fine with the 45s as is.

If funds were available now, I'd jump on the other option; Stereomour built for 45s. Maybe soon........

Steve T

[Paul Birkeland]

The big advantage available from the Stereomour is the availability of the DC heater upgrade.  For a low power 45 amp, having DC heaters will make a huge improvement in the noise floor.

Over the years, we have also developed both our cap coupled and directly coupled circuits to have large amounts of headroom in the driver circuit, which is not a possibility with a C4S load being fed from the 2A3/45 cathode.  Direct coupling with C4S loading fed directly from B+ ended up working a lot better and being much easier on output tubes (especially new production) with the soft start circuitry implemented.

If you have a 7K cathode bias resistor for the 45's, and we assume a meager current of 30mA, that will need to be a 20W resistor, which is a big old source of heat in a fairly compact amplifier. 

If you drop the primary voltage with a choke input filter, you can go to a cap coupled 45 amp with a CCS load fed from the lower B+ and end up with an excellent sounding and very reliable amplifier. 

[Paul Joppa]

I have not yet documented the technical history of the AfterGlow/ParaGlow/Paramount direct coupled amps, though I intend to do so eventually. But I do recall that the ParaGlow originally used the deYoung power transformer, and later switched to the PGP8.1. The PGP8.1 produces more voltage than the deYoung, but at the time we did not update the manual. This led to a lot of confusion at the time.

The higher voltage should be used to increase the headroom of the driver, which in the original version operated at only 100v on the plate.

You should see around 455 volts at the second power filter cap, or  440v at the 45 plate. Assuming the conservative 250v/34mA operating point, that gives 190v at the cathode, so a bias resistor  (using Ohm's Law) would be 190/.034=5600 ohms. Bias is 50v, so the driver plate should be at 190-50=140v. The driver cathode bias resistor can be increased to obtain this target. I have provided the calculations so that you can re-calculate if your powerline voltage is different from the PGP8 design target, which is I believe 117vAC.

I think you already have a working ParaGlow 45 with PGP8.1? If so, you can check the voltage (relative to ground) at the 45 plate. If it is much higher than the 440v estimate above, the adjustment will among other things require a higher-resistance cathode resistor for the 45. I suspect this may have been the case 15 years ago, which would explain the 7K resistance.

You did not identify the plate chokes, but if you do I will calculate an appropriate parafeed cap. Today, I would connect it to the 45 cathode rather than ground as in the original. There are other changes I might make now, but that's a whole 'nuther conversation, and I want to gather the history first.