PT woes: lacking a bias tap, need suggestions.

[matta]

Hi Guys,

I decided to help a friend out by using his old ELK amp and building him a Hoffman single channel AB763 deluxe reverb into the old chassis as the transformers are of the same values, I've added a choke but the PT, OT and Reverb are correct... one caveat... no 50V bias tap  

A glaring error that I missed and am now 90% done with the build  

Is there a way to bias it without a dedicated bias tap?

Thanks in advance

Matt

[FYL]

You may derive it Marshall-style from the HV winding.

[matta]

Thanks FYL, can you point me to anything specific? Do you use the HV winding to make the 50V to send to the Fender bias set-up or does it require reworking the whole bias scheme?

You may derive it Marshall-style from the HV winding.

[sluckey]

Look at this Plexi schematic...

http://www.el34world.com/charts/Schematics/files/marshall/jmp_lead_50w_1987.pdf

[kagliostro]

Something like this

Kagliostro

[matta]

Hi Guys,

Apologies for the delay, shelved it for a week to clear my head a bit. Sluckey & Kagliostro thanks for your input. Kagliostro in your drawing you show the PT as being a Hammond 270EX, which has a 550-0-550VAC, mine is 320-0-320VAC primary, due to the voltage differences I assume parts values will need to change, I'm just not in the league as most of you to work out these substitutes?

The diode and filter cap would stay the same I guess but the trimmer and resistor would need to be adapted to the 320 tap?

Also would this replace the existing BIAS trimmer in the AB763 circuit, or could I feed the output to the existing circuit?

Thanks is advance guys, I appreciate the help/support.

Kind regards

Matt

[kagliostro]

I'm just not in the league

I too

I'll try with a 220K / 250K instead of 100K

then measure the obtained range in Bias Voltage

other parts of the circuit seems fine to me

(give a look also to the posted Marshall link/schematic)

Kagliostro

p.s.: as from Mr. Merlin's book, such a way to obtain a Bias Voltage may be more safe than others because - if there is a failure in the Bias path is easy that also there is a failure in B+ and the amp still not receive B+ without Bias Voltage

[ernie_jr]

go to radio shack, get a 12.6 volt filament transformer. hook the 12.6 volt side to your 6.3 volt leads of your PT. the other side will now give you 60 volts. hook one lead to ground, the other becomes your bias tap. I use this method for all my bias power.
Good luck,
Ernie

[PRR]

> resistor would need to be adapted to the 320 tap?

Perhaps not.

The lower-volt amp also wants lower bias voltage.

What the Marshall plan does is derive about 1/10th of plate voltage as a negative bias supply. Use PT HV winding suitable for high or low plate voltage, you get high or low bias voltage accordingly. Not exact right, but there's the trimmer. 

Obviously: build it and power-up without tubes, verify the expected B+ except a bit high for lack of load, then verify that bias is *right polarity*, can be trimmed to expected bias voltage and a lot more for safe cool smoke-test.

Personally I like what Ernie is saying. Bias must NOT! ever fail. The Marshall scheme puts a brutal load on the first resistor. The 12V at 6V:60VAC plan is super-gentle on the added transformer and no great strain on anything else.

(If you can only source 240V parts there, use 240V:24V any amps and feed the "24V" side with 6VAC. That's like 1 pound of stuff in a 4 pound bag, a bit of a waste, but I bet 24V utility (thermostat etc) transformers are readily available.)

[DummyLoad]

PT as being a Hammond 270EX, which has a 550-0-550VAC,

respectfully, but no, 270EX is 275-0-275 or 550V Center Tapped  

6&1/2 american... one may be closer than you think...please forgive assumption that you're in the USA or canada if you aren't.

http://www.radioshack.com/product/index.jsp?productId=2103732  

--DL

EDIT: whoops! yer from south africa...  

http://www.hamrad.co.za/products.html  - web says they're based in cape town...

p/n 66525 - it's a 12V-0-12V to 220V @ 500mA - ignore the CT - wire it in as 24V to 6.3V fils. and you'll get ~55V across the primary leads - plenty. 

[matta]

Thanks Guys,

The smaller trano wired off the heater tap seems to be the safest option all round, I'll get one later today.

Cheers

Matt

[sluckey]

There is nothing unsafe about deriving bias voltage from the HT winding. Plenty of circuits do just that. Even the small Princeton Reverb. Although using a backward connected transformer will work well too, why would you want to use one when just a 10¢ resistor will do the same and not require any real estate?

[matta]

I picked up a 12-0-12/240VAC trano, small footprint as I was able to get a 100mA one, less real estate. Couldn't find the rating for the Bias tap off the original PT, but the specs seems to indicate the HV tap is only 150mA and the 42V tap is tapped off that winding, so my guess is this 100mA will be sufficient.

Sluckey, I hear you, had the transformer cost a fortune I would have thought twice, but i picked it up for $8USD.

Cheers

Matt

[kagliostro]

Bias consumption is very very low

Kagliostro

[matta]

Thanks all, it is working like a charm, got +-55VAC from the winding, connected it to the BIAS circuit and got -27VDC without any trimming, so should be good to go. Sadly I have some other woes with voltages, which I've made mention of here: http://www.el34world.com/Forum/index.php?topic=11158.0.

Cheers

Matt