Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LaszloS on August 03, 2010, 08:37:17 am

Title: heater vloltage too high or low?
Post by: LaszloS on August 03, 2010, 08:37:17 am

I have just put a new PT in my amp upgrading from a DR style towards a PR.  the voltage here is 115.  it is a multi tap primary 100,120,125  with the tap on 120 I get 5.8V on the preamp heaters and 4.7 on the recto.  but it is the correct (stated) voltages on the HT if I switch it to the 100V tap I get 7V and 5.3 respectively. 

which is better for the heaters 5.8 or 7?

cheers,
LZ

Title: Re: heater vloltage too high or low?
Post by: jjasilli on August 03, 2010, 09:04:50 am

Suggection:  Search the forum first.  This is a recurring topic.  I'm guessing your reading voltage unloaded (no tubes). 7VAC unloaded is correct. 

Title: Re: heater vloltage too high or low?
Post by: PRR on August 03, 2010, 12:46:21 pm

Check your voltmeter.

Either the "115V" isn't 115V, the 5.8V isn't really. or the transformer winder screwed up.

Of the three, bad winding is least likely, so check your voltmeter and voltages.

A fresh "1.5V" alaline battery should read very close to 1.56V DC. That will confirm basic voltmeter calibration. It won't confirm the meter's AC-DC conversion, but that usually works, reads half, or reads nothing, it doesn't usually get a few percent off.

Title: Re: heater vloltage too high or low?
Post by: phsyconoodler on August 03, 2010, 02:38:16 pm

In other words,get another opinion from a new battery or another meter or both.

Title: Re: heater vloltage too high or low?
Post by: LaszloS on August 03, 2010, 07:42:03 pm

I didn't think of that. I'll double check the voltmeter.

Title: Re: heater vloltage too high or low?
Post by: LooseChange on August 03, 2010, 07:45:16 pm

Just curious. What is the min/max for hearter voltage?

Title: Re: heater vloltage too high or low?
Post by: jjasilli on August 03, 2010, 09:07:53 pm

"Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher. The exception to this rule is rectifier valves, which should not be run below 10% of their rated voltage. . ."  http://www.freewebs.com/valvewizard/heater.html

Title: Re: heater vloltage too high or low?
Post by: LaszloS on August 04, 2010, 02:49:16 pm

I double checked the voltmeter with a fresh 9V battery and 9.7V also I have a collection of three cheap 9$ DMMs and they all show the same voltages

what i get is 5.7 on the heater (tubes in) on the 120 tap.  which gives me a B+ of 440 through the 5ar4 (tubes in) on the 720V winding.

this seems to be because the wall voltage is at 113VAC perhaps this is just my house I might have to try elsewhere in the city.

so long term life-wise presuming this is the normal wall voltage is it better to leave it on the 120 tap with the heaters at 5.6-5.7 or bump it up to the 100V tap and maybe try the diode business to knock the voltage on the heaters down?

Title: Re: heater vloltage too high or low?
Post by: bigdaddy on August 04, 2010, 05:36:57 pm

Who made the transformer? Were did you get it?

Title: Re: heater vloltage too high or low?
Post by: LaszloS on August 04, 2010, 09:05:49 pm

webervst

this one w022798int

https://taweber.powweb.com/store/022798intsch.jpg (https://taweber.powweb.com/store/022798intsch.jpg)