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Too much gain

Shoog

Member
2002-08-15 10:16 pm
Eire
To much gain

Hi there,
As anyone who followed my thread "looking for KT88 Circuits", will know I have recently completed a Parafeed adaption of the RH807 circuit. In its final form it just didn't have enough gain with my unity gain line stage preamp. I decided to add a ECC88 as a front end. Initially this introduced a huge amount of hum. A lot of search tracked it down to the input cable coming to close to a power transformer. It forced me to look at the grounding a lot and I am certain I have quietened it down a lot, even though before the ECC88 there wasn't much audable hum. The sound isn't quite as pure with the ECC88 added, but it has a bit more grunt.
The ECC88 has an anode load of 33K and a cathode resistor of 300K with a current of about 5mA and a B+ of about 320V. The cathode isn't bypassed, and neither is the cathode of the following ECC81 driver tube.
The only issue now is that there is just a tad to much gain. Is there any way of reducing it without using the brute force of a voltage dividing network ?

Shoog
 
if you don't like the idea of a voltage divider or volume pot after the tube stage, you can use a resistive divider in the plate resistor. Here's an example if you wanted to keep your 33K plate load, but wanted half the voltage gain: Take a 15K resistor from B+ to where the output coupling capacitor connects, and then from this node take annother 15K resistor to the plate. You essentially have two resisotrs in series as the plate load, and tap the signal in between them. It's a resistive attentuator, but it doesn't add any extra stuff to the amp pretty much.

Personally, if I had extra gain o burn, I'd add a volume pot and possibly a set of Baxandal tone controls.
 

Shoog

Member
2002-08-15 10:16 pm
Eire
"if you don't like the idea of a voltage divider or volume pot after the tube stage, you can use a resistive divider in the plate resistor. Here's an example if you wanted to keep your 33K plate load, but wanted half the voltage gain: Take a 15K resistor from B+ to where the output coupling capacitor connects, and then from this node take annother 15K resistor to the plate. You essentially have two resisotrs in series as the plate load, and tap the signal in between them. It's a resistive attentuator, but it doesn't add any extra stuff to the amp pretty much."

Sounds like a good idea. I really don't want a volume pot on this "power amp", my preamp does that. Halving the gain would also halve the residual hum.
It tragic that the input stage is intoducing any hum at all because the Constant Current loaded output stage is one of the quietest output topologies know. If the front end is done perfectly then this baby should be wisper quiet.

Shoog
 
Another way to tackle the excessive gain is to change tube types. Replacing the the 'T7 driver with something from the 'V7 family, say the 5965, should do the trick. You go from a mu in the 60s to a mu in the 40s, while the remaining characteristics are quite close.

On the hum front, you could deal with pickup by trafo coupling to a difference amp at the I/P. The trafo breaks any ground loop, while the difference amp rejects common mode hum that was picked up. DC on the small signal tube heaters is yet another technique available for reducing overall hum levels.

BTW, another potential advantage of a difference amp at the I/P is the ability to add absolute polarity switching conveniently.
 

Shoog

Member
2002-08-15 10:16 pm
Eire
Would the V7 series work as drop in replacements for the ECC88. If so I would look at them for future replacements.
I am at this stage very reluctant to start pulling this thing to bits, as since I have tied everything down properly I have tiny amounts of hum (the input choke buzzes louder), and it works.
I can live with the slightly excess amount of gain. Experience has shown me that when you start pulling working things about they tend to go up in smoke.

Thanks

Shoog
 
The 'V7 family has a mu in the 40s. The 6DJ8 family has a mu in the 30s. Replacing the ECC88 with a 5965 would INCREASE gain. Also, the electrical parameters of the 2 families are very different. Yet another thing to consider is the SUPERIOR noise factor of the 6DJ8 family; it makes for good S/N performance.

You have a good chance of success, without any other changes, pulling the ECC81 (12AT7) and inserting a 5965. Surf over to TDSL (http://tdsl.duncanamps.com/tubesearch.php) and compare the data sheets for the 12AT7 and the 5965.