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vanderveen's 30W PP amp..

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alex278

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Joined 2005
#1
As my first DIY project, I built the 30W push-pull amplifier with 2xEL34 per channel as described in Menno vanderVeen's book on transformers for tube-based amplifiers. I did make some mistakes in building it (some because of the layout in his book, which isn't exactly clear, some of my own, such as connecting the power transformer backwards to the mains 😱 ).

However, it seems to work more or less OK now. The only thing is that it sounds too bright, and that the bass is tight, and sounds 'strange'. As if it is mixed out somehow.

I've used some of the older amplimo output transformers (XC462), not the dedicated VDV8020 as described in his book. Could this be the reason for the particular sound of this amplifier?

(Another thing that also amazes me that it makes almost no difference if I convert the wiring of the EL34's from triode to pentode; I can hear that there are some differences, but it's very subtle. Not what I expected from the ravings of the tube-culture..)
 
#2
Originally posted by alex278 ...However, it seems to work more or less OK now. The only thing is that it sounds too bright, and that the bass is tight, and sounds 'strange'. As if it is mixed out somehow.

I've used some of the older amplimo output transformers (XC462), not the dedicated VDV8020 as described in his book. Could this be the reason for the particular sound of this amplifier?

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if you could post a schematic the gurus might be in a better position to help.

also from the little i have learnt the quality of OPTs is really important with these amps so that could be the difference. once you have the amp sorted out the triode-pentode differences might be more apparent.
 
#3
Yep, need a schematic

I never had faith in any EL34 for home entertainment amps. Perhaps a conversion to the older 6L6G, KT66 or 6550 family of tubes? Preamp tube side can trash an amp as well. Components are as important too.
 
#4
Hi

Probably Alex is referring to something very, very close to this design
http://www.plitron.com/PDF/PB/Article/Atcl_9.pdf

I read vanderveen's book to and if I'm not mistaken (it was a long time ago) he uses the same driver/phase splitter stage (shown in the pdf) to drive PP EL84, PP EL34, PPP EL34 and PPPPP EL34 (8 times EL34 in PP for 100W in triode).

And Alex, are you sure he uses the 8020 for the PP with EL34? This trafo has 8k plate to plate and can handle 20W. As far as I know a PP EL34 needs lower plate to plate resistance and, as you mention, will give higher output than 20W the 8020 can handle. From what I remember the 8020 is used in a EL84 PP and in the Maurits.

Erik
 
#5
Re: Yep, need a schematic

amperex said:
I never had faith in any EL34 for home entertainment amps. Perhaps a conversion to the older 6L6G, KT66 or 6550 family of tubes? Preamp tube side can trash an amp as well. Components are as important too.
Click to expand...

Sounds like what's going on here is much more fundamental than voicing with different tubes (assuming each is used with appropriate values). I have some el34 amps which sound very fine to my ear - certainly nothing like what is described here. So it's certainly possible to make at least a decent sounding amp with those. I wonder if the balance in the opt's is way off. Are toroids used for opt's and operating under saturating conditions?

Sheldon
 
#6
Yes, the OTP balance is of the most importance with toroid transformers to avoid saturation and degraded output in the bass region.
Try to let the amp warm up min. 30 minutes and then adjust the Bias as close as possible.
I'm building a fully balanced 50W triode amp with four 6550 tubes in the output and a Plitron PAT-2100-CFB/H and with only 20mA unbalance in the OTP the bass is sounding very thin and distorted. I have build a beta version of a Bias servo so the error can be within 1mA.
 
#10
Some further info:
1) the output transformer I use is not the amplimo XC462, but an older, earlier type, made by ILP, which doesn't have an ultralinear connection.

2) the primary inductance of this transformer is 'only' 16H (at 120Hz; 7H at 1Khz), whereas the primary inductance of the vdv3070 should be about 500H (at least, that's what I suppose; now I look at the datasheet again it says in a footnote 'maximum value, measured over secondary, transferred to primary'. I can figure out what he means with 'maximum value', but what does he mean with 'transferred to primary'??

(later this week more measurements)
 
#12
BW, I tried measuring the frequency response of this transformer, and that's an interesting job. If I hook up the primary directly to the 50 ohm output of my function generator, connect the secondary to an 8 ohm load, and measure the voltage over the 8 ohm resisitor, I get a nice dip at about 3 KHz, and a drop-off which starts at about 50KHz. Is the 3KHz dip due to the capacitance of the transformer itself?

(I'll recheck it this evening, because it might very well be possible that I'm measuring the wrong thing..)
 
#13
I'm planning on making the 100 watt ver, and was wondering if the power transformer would be able to supply the B+ current for 2 of the amps, or will I need a separate power supply for each amp??

I'm planning on using a 6.3v 20 amp transformer I have for the filaments.
 
#15
Further update: I've discovered that the power transformer is underdimensioned for this amplifier -- I use a 300 V transformer, but the voltage drops too much as this transformer can't handle the load of 2 EL34's -- it is specced for 100/150mA. The only other transformer I currenly have is a 360 V transformer@600mA, which gives a little too much output (when I plugged it in my EL34's got red-hot 😱 )

So, I'm quite curious if the 'bright' sound will go away if I use a proper trafo..
 
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