Steve Morley,
Good point about the lightly loaded power transformer.
Less B+ current, Less Filament current causing less DCR drop, and less drop in the primary DCR due to the lower primary current.
It is hard to say how many percent (%) the filament voltage will increase.
The Six Pac is rated for 117V/234V.
I often get 123V at my power outlets. That is 5% more than 117V.
Tube Life under those conditions?
I adjust my filament voltage to be much closer than 5% to the ideal rating.
Good point about the lightly loaded power transformer.
Less B+ current, Less Filament current causing less DCR drop, and less drop in the primary DCR due to the lower primary current.
It is hard to say how many percent (%) the filament voltage will increase.
The Six Pac is rated for 117V/234V.
I often get 123V at my power outlets. That is 5% more than 117V.
Tube Life under those conditions?
I adjust my filament voltage to be much closer than 5% to the ideal rating.
That's easy to solve. Just add some resistor in series. For the input tube that supplied with DC voltage you can actually filter more.
The problem is when you add tubes and don't have enough headroom...
With one or two pairs one other thing to check is the inductance of the anode voltage supply. Being input choke filter with just 2H this will not be enough with approx. less than 200 mA draw as the critical inductance will be higher.
The circuit needs to be reviewed overall. I assumed all these things were obvious. If not, it's better to leave it as is and build another small amp with EL84's or similar for the summer hot days with the money one would spend for 6 pairs of good quality EL34's.....
The circuit needs to be reviewed overall. I assumed all these things were obvious. If not, it's better to leave it as is and build another small amp with EL84's or similar for the summer hot days with the money one would spend for 6 pairs of good quality EL34's.....
Last edited:
Yeah, that makes sense in the end. It's not worth all the hassle if I have to modify the circuit in any way to achieve good results.
One thing I didn't get though. Steve Morley and 6A3sUMMER were mentioning a higher filament voltage due to a lighter load on the power transformer. I'm not sure I understand why that would happen, as from the schematic it seems filaments are run from a separate winding and they're in parallel so they shouldn't be affected by any change in B+ through my logic. Maybe I misunderstood something.
If you give up two pairs of tubes, you will have a power load of about three times lower and the THD will increase considerably, as Raa is no longer optimal for a single pair of tubes. You can do and the next "juggling": on the 8 ohm side you connect the 4 ohm or 16 ohm speakers to connect the 8 ohm speakers, this way you will "fool" Raa reflected. I did not explain it too technically, but you can understand! Of course the power will be just as small but the THD will remain the same (at the power you have the same time). The voltage at the filaments may increase somewhat if you remove the four final tubes out of operation if the power transformer is not potent enough, anyway I do not think it will grow dangerously long!
[In cazul in care veti renunta la doua perechi de tuburi din etajul final sigur veti avea o putere pe sarcina cam de trei ori mai mica iar THD-ul va creste considerabil intrucat Raa nu mai este optim pentru o singura pereche de tuburi.Puteti sa faceti si urmatoarea "jonglerie": pe secundarul de 8 ohmi conectati boxele de 4 ohmi sau pe cel de 16 ohmi conectati boxele de 8 ohmi,in acest fel veti "pacali" Raa reflectat.Nu am explicat prea tehnic,dar poate ati inteles! Bineinteles ca puterea va fi la fel de mica in schimb THD-ul va ramane cam acelasi (la puterea respectiva pe care o aveti de aceaata data).Tensiunea la filamente s-ar putea sa creasca oarecum daca
scoateti patru tuburi finale din functiune mai ales daca transformatorul de alimentare nu ar fi suficient de potent,oricum nu cred ca va creste periculos de mult!]
If it is wrong to write in the language of our colleague, I apologize and delete it!
Last edited:
You get the 6.3V at 9A current draw for the 6 EL34's + all the other supplies from one single transformer. If no load (i.e. no EL34's, just the DMM measuring) the voltage will be higher than 6.3V. How much higher depends on the transformer design. Because the power transformers are often designed to be as small as possible regulation is never exceptional. Say the heater voltage is 8V with no-load. This means that for a current draw between 0A and 9A you will get something between 6.3V and 8V. So you should check if the actual heater voltage is within 5% of 6.3V when you remove one or two pairs. Otherwise tube life will suffer. Same thing for the other voltages....
I think I understand, but there's not much I can do, as I have a 6ohm pair of speakers, connected to the 4ohm tap.
Oh, ok... that makes sense. My mind was stuck thinking about regulated supplies for heaters. Thanks for the explanation.
Watch this videos
About this Amp
Repair and modification
Repair and mod
Two Six Pac's On The Bench - YouTube
Amplifier Troubleshooting and Modification. - YouTube
About this Amp
Repair and modification
Repair and mod
Two Six Pac's On The Bench - YouTube
Amplifier Troubleshooting and Modification. - YouTube