unusual biasing

ask your general tube related questions here

unusual biasing

Postby dhuebert » Thu Nov 04, 2004 8:46 am

I was given an AGS stereo receiver recently and discovered an interesting biasing scheme for the output tubes (6BM8s push pull). The cathodes of the four 6BM8s are wired in parallel with one end going thru the heaters of a pair of 12AX7s, which are the phono preamp section, and then to ground. So the 12AX7 heaters in series provide the DC biasing for the 6BM8s. The reason I started investigating this is because one of the 6BM8s would glow red hot when I switched the unit on. I tried swapping tubes but this didn't solve the problem. I looked at voltages around the offending tube and found the plate voltage was about 20 V lower than the rest, so I thought maybe a problem with the transformer. After about the fourth power cycle, everything seemed to work with no repairs done. The only problem was now there was about 8 V on the 12AX7 heaters, not enough to run them. Suddenly the heaters came up and a different 6BM8 started to glow. Thereafter a different tube would glow everytime it powered up. I'm not really interested in repair as I will be using the receiver for parts, but I wonder if anyone would like to comment on this.
On a prevous post I talked about an AGS PA amplifier. The power supply is one long daisy chain of resistors and caps with the result that voltages to the last tubes were very low and dropped with the slightest load, not very good design. I spoke to a fellow about it who used to have a repair shop, and he assured me that AGS was crap. Judging by the number of repairs to the receiver I have, and the quality of construction, I would have to agree. However, it has 16 tubes and some other goodies so will make a good organ donor.

Don
User avatar
dhuebert
KT88
 
Posts: 820
Joined: Thu May 01, 2003 9:26 am
Location: Winnipeg Manitoba Canada

Postby erichayes » Thu Nov 04, 2004 3:47 pm

Hi All,

This kill-two-birds-with-one-stone scheme was used primarily by the Japanese manufacturers, although the first time I saw it was in a Bell 3DT made in 1958. Fisher used the preamp heaters in their bias supplies in the 400, 500 and 800 series of receivers, among others, but as part of a voltage divider for a separate negative power supply, rather than as a cathode resistor for the output tubes.

The main problem with this configuration was that everything had to be just right. With 6BM8s, the total cathode current for 4 tubes is around 150 mA with 250 volts on the plates. Two 12AX7s in series draw 150 mA, but need 24 nominal volts to operate. 6BM8s, however, only want to see a 16 volt differential between the control grid and cathode at 250 Vp in class A, so compromises had to be made. Usually, the plate voltage was raised to the point where cathode voltage would be around 20 V (a shunt resistor would also be placed across the 12AX7 heaters). Unfortunately, that would take the cathode current dangerously close to the 5 watt maximum plate dissipation rating of the tubes. If one tube failed to pull its weight for whatever reason, the other tubes would superconduct. Usually the tubes were unmatched, so one tube would draw the lion's share of the current, resulting in... guess what?... a red-hot plate. All it takes is a dirty tube socket to cause this problem, BTW.

I have an Olson receiver with this tube and bias setup on the back burner for installation of a negative voltage bias and heater supply, a l? Fisher. Since this thing's no collectors' item, I don't feel guilty in the least about foresaking originality for reliability and improved fidelity.
Eric in the Jefferson State
erichayes
KT88
 
Posts: 987
Joined: Fri Jan 23, 2004 9:01 pm
Location: McKinleyville CA

HF87

Postby Shannon Parks » Thu Nov 04, 2004 5:05 pm

The Eico HF87 used this method, too. Sometimes engineers are too clever for their own good. ;)

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3764
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Postby erichayes » Thu Nov 04, 2004 7:14 pm

Not clever, Shannon; cheap.

I don't have anything to bear this out, other than my experience with manufacturing, but I'll bet the HF-89 was developed first, and the '87 was brought out to go head-to-head with the Dyna Stereo 70. That meant getting rid of anything that cost more than it was worth sonically. This is where an engineering-bean counter mutant called a "production engineer" would enter the game and start raping the design engineers' plans. The PE's credo was "Cheapen it up until you hear a difference, then go back one step". You also have to remember that this was a time when tube manufacturers were in the business of selling tubes rather than maximizing their lives, so it was in everyone's best interest (except the customer's) to make circuits that would wear out tubes.
Eric in the Jefferson State
erichayes
KT88
 
Posts: 987
Joined: Fri Jan 23, 2004 9:01 pm
Location: McKinleyville CA

Postby dhuebert » Fri Nov 05, 2004 8:17 am

If one tube failed to pull its weight for whatever reason, the other tubes would superconduct. Usually the tubes were unmatched, so one tube would draw the lion's share of the current, resulting in... guess what?... a red-hot plate


Thanks guys, I thought it might be something like this.

Since this thing's no collectors' item, I don't feel guilty in the least about foresaking originality for reliability and improved fidelity.


I don't feel guilty in the least about forsaking originality for something that works. With that in mind my TR6 gets more compression, headers, hot cam, sway bars, 5 speed, etc, etc. Although my 1959 Canadian export standard beetle will go back to factory...

But I digress.

Don
Last edited by dhuebert on Fri Nov 05, 2004 11:59 am, edited 1 time in total.
User avatar
dhuebert
KT88
 
Posts: 820
Joined: Thu May 01, 2003 9:26 am
Location: Winnipeg Manitoba Canada

Postby Shannon Parks » Fri Nov 05, 2004 10:55 am

erichayes wrote:Not clever, Shannon; cheap.


Agreed.

But a lot of EE's consider clever and cheap the same thing. For instance, my dad was a huge proponent of membrane keypads when they first came out. Cheap, reliable and weatherproof - nevermind that people <love> tactile response. Remember the Sinclair Z80? I still haven't won dad over on tactile keypads.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3764
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Postby DortoH » Fri Nov 05, 2004 12:46 pm

I still curse the keyboard on the Z80.
DortoH
 
Posts: 100
Joined: Mon Aug 30, 2004 1:00 pm
Location: Great White North aka Canada


Return to tube 101

Who is online

Users browsing this forum: No registered users and 10 guests