im a newbe here , this is my first post but had my first hi fi in 1950! I have beeen lurking here for a month and read ALL this thread about 10 times lol. .well i recieved my 12au7 bravo yesterday. im listening to it on my senns 580's . ive had them 15 years but never heard them on a head amp before and they always sounded good, but not this good! the chinese tube sounded somewhat closed in and a trifle harsh, so i put in an RCA CLEAR TOP and with 2 hours on it it is spectacular. i listen to classical , listening to "moussorgsky, pictures at an exhibition ", and ive honestly never heard it as good as on this little piece of crap . I also played a mozart piano concerto no. 27 and the piano sounds pristine , even on the top end . mine is upgraded with a real bat handle on -off switch. im eventually going to make OOHMS and Solderdudes mods but honestly at this moment ill enjoy it for what it is for awhile. I have one problem. it occasionlly stops playing for maybe a 10th of a second , then comes back on singing! what the hell? ive checked all the connections and alls good. this is one of the best sites out there , no bull s**t and i am delighted i can be here. my heart felt thanks to all members for a FINE thread.
well i figured out why the sound was cutting out ( i think ) my 15 year old senn 580's finally developed an intermittant at the plug where it plugs into the phones. if headphones will last 15 years for me , they will last a lifetime for someone else ;D i guess ill either have to hold my head real still ;D or try to fix them . im 81 on a fixed income so replacement isnt an option they still beat most phones out there. regards , seaton
If it isnt baroque, fix it !
PS. the right mosfet heat sink is too hot to touch and the left i can hold my finger on it for 15 seconds so i guess the first mod will be trim pots for bias and change the resistor to reduce v on the tube . my amp is dead quiet ( as long AS IM NOT ON MY CELL PHONE ;D. pecularly the cross talk is minute and just acts like cross feed in the phones. the sound stage sounds a little less "inside the head" compared to what ive been used to for 15 years
I've done all the mod's on my Muse Audio version of this hybrid tube amp. All is well, got bias tuned and a nice big fin for the LM317AT's. I'm very happy with the quality, but I'm having some problems with the volume getting very high just by slightly turning up the volume from zero. I've de-soldered the pot and measured it to ~85Kohm's, my output resistors are 68ohm's pr. a tutorial on the different mods for this amp. Could i put some resistors on the pot leg's or would it be better to switch it for a 180K-250K pot ?
EDIT: Forgot to ask, or skip changing the pot and put in bigger output resistors?
Thanks for the suggestion, but it didn't work. The volume pot is 85K, I put in 8.2K since I didn't have any 8.5k on hand, but the only difference was that I could hear some audio coming through while the volume pot was set to the lowest setting.
An 85k pot does not exist. 100k does exist but as far as I know all these amps are fitted with 10k lin volpots. (10 3B markings on it see picture below)
You can also measure the (DC) input resistance by measuring across the input RCA with the amp turned OFF.
When the extra resistor is fitted correctly there should not be any audio leaking so I suspect somethink wasn't done correctly.
This is how it should be done.
Of course every positive has a negative. The vol pot will act a bit more log type like (certainly not exactly the same) but the drawback is the input inpedance of the amp (normally around 7kOhm for AC signals, 10k for DC signals will vary depending on the volpot setting. Fully counterclockwise the input resistance of the amp will be 7kOhm. at 12 o'clock it will be around 5kOhm and fully clockwise it will be just below 1k.
It is a good method to see if you need a log volpot.
In this case replace the volpot for a log type. Value may be a 5k pot, 10 k pot and max 20k pot.
note: the pictures used for this illustration were taken by Oohms
I'm noticing that cross talk, even after the modifications, is still quite audible. To test it I unpluged one RCA cable for Left channel and listen to Right channel - you can hear a fairly loud sound that's coming from the right. Does anybody know how to remedy the issue?
Also, any suggestions on how to convert the amp to support 6v tubes - with a switch? I tried a couple of mods that I came up with - it worked, but for some reason voltage on the heater was dropping very low, so the tube would take 2 minutes to warm up - this might be the result of more demand for heater current with 6GU7. There are some fantastic tubes in 6GU7 family (like Amperex) that are selling right now practically for nothing.
Yah. right now the amp is back in the 12v mode. However the cross talk is definitely there. It's not strong, but if you crank up the volume to max with L RCA plugged in, you can definitely hear music in R channel - not loud, but at listenable volume. If I understand the schematic, the mod will reduce cross talk, but the channels are still connected through the heater, which works like a 20 ohm resistor.
BTW, you are referring to the TSR heater mod, could you please point me to it - Google returned just garbage.
In 12V mode with the link between pin 4 and 5 removed there shouldn't be crosstalk (other than that by the tubes themselves). The heaters are not connected but in the center tap connected to ground so essentially each channel only feeds 1 heater.
In the G2 tutorial you can also deduct what needs to be done to convert the 12V to 6V.
Last Edit: Apr 19, 2012 22:17:25 GMT by solderdude
Ahh. You are so absolutely right, solderdude. So it's even stranger (before I could at least logically explain to myself that I can hear the Left channel in the Right one with right RCA completely disconnected. Do you hear absolutely nothing on your amp with volume cranked up all the way?
Thanks, solderdude. I'll try it tomorrow. The amp is at work, so I don't have access to it. I do have some cheap headphones connected to it - that might explain the issue. I will experiment with my HD650 and see if the cross talk is still there.
The reason is I cannot adjust the bias voltage with that 2k2 trimpot, so I replace the 2k2 resistor with bourns trimmer that I have on hand. And it works, I could adjust the bias and the amp give nice sound. Is it ok to do that? FYI I can solder but I know nothing about how electronic parts and circuit is working. This is my first project after 20 some years ago made an AM radio at 7 grade. But I think I'm gonna remove the 2k2 trimpot and using only the bias adjustment on anode side.
Can you explain the technical difference between the 2 methods bias voltage adjustment? Which one is better? I use IRL530 and set the bias voltage at 13.5v, set the cathode pot to middle position and the multiturn pot value is 30K and 26K.
cathode resistors (pot in this case) make a form of 'autobias' in normal tube amps (higher voltage amps) because the anode current wants to rise to it's maximum current when there is no voltage across the grid and cathode. When it heats up current starts to flow which causes a voltage drop over the resistor. This in turn 'dials' back the current so it 'stabilises' drawback is the added resistance which forms a highfrequency cut-off point (can be counteractet with cathode caps) but the pre is the tube ends up in a more linear part of it's performance so has a better 'fidelity' at higher amplitudes. The higher this resistance the lower the current, the higher the noise level.
So some resistance is wanted and a higher current as well. There is an optimum, don't know where.
Set the cathode pots to 1/4 or 3/4 and adjust the anode current again if you experience noise. If not, leave it as is.
May be your feeling but technically and measurably things have actually changed. The frequency range has expanded further into the hypersonic range. Your dog or pet bats may appreciate it if your headphones would be able to reproduce it and hyper sonic relevant info would be in the recording (not in CD).
All that matters is you enjoy the fruits of your labour, mindset or real... who cares as long as you enjoy music.
Last Edit: Apr 24, 2012 10:27:29 GMT by solderdude
measurably it might, sonically depends on other factors such as routing of wiring, decoupling, layout and the subjective ingredient. Biggest improvement that can be had is by taking the heaters out of the audio path. this way the current sources always work as they are supposed to. Now from a certain point onwards on one half of the sinewave the don't. This would involve a separate power supply for the heaters though.
Last Edit: Apr 24, 2012 11:33:44 GMT by solderdude