DIY Dyna MK 2's and using bottlehead C4S - diyAudio
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Old 17th April 2005, 02:49 PM   #1
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Default DIY Dyna MK 2's and using bottlehead C4S

Hi, Im looking to improve 2 peices of tube gear and have just purchased a C4S kit and want to know what stage of either unit might produce the best bang for the buck. This is what I own.

1. Lazarus Cascade Basic Preamp. I have owned this since 1988 and is a 12AX7 based preamp. I use the line stage and the 1 stage is gain and the 2nd stage of the 12AX7 is a direct coupled cathode follower output.

2. Dynaco Mark 2 amps with the following mods. Updated front end with VTL driver. 1st stage is Parallel 12at7 and 2nd stage is 12at7 long tail pair. It has ss rectification and 1300 mf outboard capasitor bank as well as choke. It also uses a quad set of el34s wired in triode and filiment transformer is outboard. The output is biased to 25 ma per output tube. The units make about 37 watts per block using the 16 ohm tap to drive 8 ohms.

Thease amps sound nice but have one tecnical glitch that I have no answer. When I veiw the output on my O scope the output gets triangled when Im above half power ( 17 watts ) and the output has a load and the freq. is above 13K. It did this in ultralinier mode also with one set of el-34s ? when output is unloaded it clean as can be. Is this inductance of the output transformer. Any fixes?

Sorry to get off track.

Thank you for any replys. Fred
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Old 17th April 2005, 07:36 PM   #2
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Put the CCS on the cathode follower...

dave
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Old 18th April 2005, 03:00 AM   #3
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Dave, You think the CCS is best placed in the preamp.
Also any thoughts on the question of why I see the waveform of my poweramp change as the freg. is raised?
Fred
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Old 18th April 2005, 03:18 AM   #4
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Quote:
Originally posted by freddymac406
Also any thoughts on the question of why I see the waveform of my poweramp change as the freg. is raised?
That will take someone with more practical experience to answer.

dave
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Old 18th April 2005, 05:49 AM   #5
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Fred,
The "Triangling" of the waveform is typical power bandwidth problem behaviour. That is: no high frequency response at higher power.

The EL34 in Ultralinear has about 4 to 5K rp so for a single pair say a Source Resistance (=2rp) on the primary side of around 9K.

The EL34 in Triode Mode has about 1500 Ohms rp so 750 Ohms each side and a total Source Resistance of 1500 Ohms for a Quad.

This means that you should be driving primary capacitance and leakage inductance of the transformer much more effectively with quad of triode strapped EL34s than with the single pair of Ultralinear connected tubes.

I notice you said that you attach your 8 Ohm load to the 16 Ohm tap to attempt to address this Source Resistance change BUT you've got it backward. This will make the problem worse.

Try attaching your 8 Ohm load to the 4 Ohm tap instead and let us know if that helps.

Cheers,
Ian
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Old 18th April 2005, 10:12 PM   #6
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gingertube,

The Dyna MK2 only had 8 and 16 ohm taps. Can you recommend a output transformer that would be a good canadate that does not Saturate with power and freq.
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Old 19th April 2005, 12:22 AM   #7
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gingertube ,
With how the spectral balance of music is , how much power do you need?
Is this typical for dynaco mark 2 output transformers
and tube amps in general .
With everything said lets not talk about square waves.
fred
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Old 19th April 2005, 12:45 AM   #8
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I got the last part of the above WRONG - trying to dash off a reply after quiting time but before I left work. Attaching the 8 Ohm load to the 16 Ohm tap is the right thing to do to reflect a smaller load to the primary to match up with the reduced source impedance.
I'm a bit hesitant still, to simply say abandon your MKII Output Trannies. As I said (and got right) in the first post the reduced source impedance of 4 triode strapped EL34s should better drive the existing trannies. Before proceeding with that extreme do a few more checks looking for a high impedance at high frequencies in the tube/output transformer/power supply loop.
Is the amp cathode biased? YES - then check/replace the bypass cap.
I noted that you said that there is a choke in the power supply. Is there plenty of capacitance on the ourput transformer primary centre tap side of the choke - that is, did the capacitor bank get connected to the wrong side of the choke by mistake? Check these sorts of things first.

The fact that you said it was the same with a pair of EL34s in Ultralinear concerns me - it should be significantly better with 4 x EL34 in triode mode.

If you are finally convinced that it is your output trannies then:
With 4 x EL34 in triode mode you will get maximum power with Raa of around the 1500 Ohms. Allow a bit higher to account for losses and a bit higher again if you want minimum distortion and are prepared to drop a small amount of power. There are transformers available form just about ALL of the major manufactureres with Raa of 2200 Ohms designed to be used with quads of EL34,KT88,6550or 6L6 etc. If you don't want to spend a lot of cash then maybe a Hammond 1650T will suit you. If you do want to spend a heap of cash then a Plitron VDV-2100 will do. Both of these are actually rated at 120W which is more than you need. Check out Magnequest in the US and Lundahl in Europe - both should have decent offerings.

Cheers,
Ian

Just saw your second post in time to get in and edit.
The 35 to 40 Watts you'll get from 4 x EL34 in Triode Mode is ample for just about any speakers / room size / listening level etc.
AND the MKII Trannies should be able to handle it.
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