Yet another completed 100W Non-HRM (with a Biasing question)

[Guitarman18]

Here is a picture of my recently finished 100w Non-HRM. This is my second amp in the D'clone style. The first one was a SE 6V6 based amp made mostly from spare parts I already had, to see if I had it in me to go for the full-on 100 Watter. For this build I managed to get one of Funks chassis', which is absolutely beautiful and is really the 'icing on the cake'. After having checked everything, I powered it up on my variac and all voltages checked out within ballpark, so I crossed my fingers and powered her up for real. To my relief, astonishment and surprise there were no issues...... Except my bias reading.

My 6L6's are reading 444volts on pin 3, but my bias pot range only goes from 45.3- 85.6 ma. According to my Torres book, it says I need to be in the early 30's. If anyone can help me sort this out I would really appreciate it. I've used Funk's 'Correct Modern Bias Circuit, however I put in a 3K resistor instead of the 3.9K before the reversed diode. Is this the culprit or should I increase tail resistor? (27K at present)


I would like to take this opportunity to thank all the members who have contributed to the wealth and quality of information available on this forum. Particular thanks must go to Brandon for kindly answering so many of my questions and being so gracious when supplying my chassis' etc. Also I would like to thank Gil, Dogears, heisthl, Bob-I and all the others that have directly responded to my plea's for help. Without their contributions, I would not have been able to do this.

Cheers, Paul.

[Bob-I]

You'll need more -bias voltage so increasing the tail resistor is the way to go.

Nice clean work. I'll bet she sounds great.

[heisthl]

It alright to decrease the input resistor to 1k - if it's easier.

[JamesO]

Great job!

[Structo]

Just curious, why the doubled up bias point resistors on V4 and V5?

[tubedogsmith]

Just curious, why the doubled up bias point resistors on V4 and V5?

Just a guess but it may be the ground reference resistors for the heaters.

[Guitarman18]

heisthl wrote:

It alright to decrease the input resistor to 1k - if it's easier.

Thanks heisthl, I've changed the input resistor to 1k, which gives me a bias range of ~20-60ma range. I tried to use 2.2k but that only gave me a minimum of 35ma. I've now got 460v on pin 3, so I've set the bias @ 31ma. Is that around where I need to be?

Bob-I wrote:

You'll need more -bias voltage so increasing the tail resistor is the way to go.

Nice clean work. I'll bet she sounds great.

Thanks for the advice Bob, I went with the input resistor though because of ease of access. Now that I've swapped out the 150K before the OD ratio pot for 180K, she is sounding great. For me, it seems to give it more of the D*mblesque qualities I was expecting.

JamesO wrote:

Great job!

thanks JamesO, best of luck with your current build, keep us posted.

Structo

Just curious, why the doubled up bias point resistors on V4 and V5?

As tubedogsmith suggested, they are the ground reference resistors for the heaters coming off pin 7 of V5 and pin 2 of V4. I've used Dale 2w 1% 100ohms. Next to them, on pins 1&8 on V4 & V5 I've got 1ohm Dales for the bias points.


Cheers, Paul.

[heisthl]

Many like 32MA I like 35MA.

[Structo]

Here is a handy bias chart that shows plate voltages and bias numbers.

I don't know how valid it is but I thought I would toss it out.

http://members.shaw.ca/house-of-jim/Htm ... ables.html