|
Home | Forums | Rules | Articles | diyAudio Store | Blogs | Gallery | Wiki | Register | Donations | FAQ | Calendar | Mark Forums Read |
Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum |
|
Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving |
![]() |
|
Thread Tools |
![]() |
#31 |
diyAudio Member
Join Date: Aug 2005
Location: Spokane
|
Here's a post on this from years ago by Mike at Magnequest regarding his rule of thumb for calculating an acceptable bias current for an unknown transformer.
RE: Looking for a output trannie match, Scott LK72 - mqracing - Tube DIY Asylum |
![]() |
![]() |
#32 |
diyAudio Member
Join Date: May 2002
Location: Kiev, Ukraine
|
Correction to my previous post: 56mA is the cathode current, not the plate one (forgot about g2).
Plate current is 44mA, g2 current is 12mA, DCR is 250R (each half), so extra 3.1W of heat at 90mA and 22.5V drop. Not that great indeed. |
![]() |
![]() |
#33 |
diyAudio Member
Join Date: Apr 2015
|
There is a saying here in germoney:
whoever buys cheap buys twice |
![]() |
![]() |
#34 | ||
diyAudio Member
|
Quote:
Quote:
So with Ip = 0.09A per winding, each winding (half of the primary) needs to have DCR of no more than about 220 ohms. 220R(0.09A) = 19.8V dropped across the winding The power dissipated would be 0.09squared(220R) = 1.782 watts (hot!) So it looks like 220 ohms per primary half would be marginal. Better to aim for 150 ohms or less. Is this achievable? I don't know... I need to measure some OPTs. -- A pair of 8k:VC OPTs meant for Push-Pull EL84 measure about 200R per primary winding, or almost 400R from plate to plate. I guess those would be marginal for Ip = 90mA per tube. -- Last edited by rongon; 24th November 2020 at 10:56 PM. |
||
![]() |
![]() |
#35 |
diyAudio Member
|
Another push-pull OPT I have, with a 5k primary rated for 30W and 75mA per tube max, measures 85R per primary winding (170R plate to plate).
Specs for the Dynaco MK-III Output Transformer A-431: 4.3K Push-Pull with 33% Screen Taps to 4, 8, & 16 ohms 60 Watts Frequency Response: 10 Hz to 100 kHz +/- 1 dB @ 1 Watt Primary DC Resistance: 142 ohms Primary Inductance: 40H @ 120 Hz Hmmm, only 142 ohms. I think that's plate-to-plate too. So it does appear that a 50 or 60 watt OPT will work best for my application. The thing is, I remember what the Dyna A431 OPTs sounded like. I tried using them for a pair of 300Bs and 2A3s. They sound kind of big and dopey compared to something like the Dynaco A470 (ST70) or the OPTs from a Fisher 800C (why did I sell those off??). My suspicion is that big honkin' OPTs like the Dyna A431 have that big transformer thickness to their sound. Unfortunately, that's what's going to be required to run GU-50s safely with 90mA quiescent Ip. Am I right, or am I on the wrong track here? Transcendar's 30W 6.6k OPT specs are: 6.6K Push-Pull to 4, 8, &16 ohms Frequency Response: 10Hz to 100 kHz +/- 1 dB @ 1 Watt Primary DC Resistance: 235 ohms Primary Inductance: 65 H @ 120 Hz Weight: 4 lbs So 235 ohms plate-to-plate and 4 lbs each. Those are a lot lighter in weight than Edcor's 30W OPTs (6 lbs each). I wonder what that means... It looks like if I want some headroom for high current swings, I may need a pair of big boy OPTs. Perhaps 50W rated. -- Last edited by rongon; 25th November 2020 at 12:02 AM. |
![]() |
![]() |
#36 |
diyAudio Member
|
It seems Hammond Mfg gives max current ratings per each half of the primary for their push-pull OPTs.
I copied a table from their catalog, listing their PP OPTs. Their part no. 1620 is rated 20W, 6.6k plate-plate primary, max Ip is 158mA per side. Weight is 3.5 lbs. Part no. 1650H is rated 40W, 6.6k p-p primary, max Ip per side = 200mA. Weight is 6.5 lbs, which is similar to the Edcor CXPP45 and CXPP50 OPTs I was looking at (those are 6.75 lbs). Part no. 1650P is rated 60W, 6.6k p-p primary max Ip per side = 200mA. Weight is 8 lbs, which is pretty darn heavy. So... What does that tell us? I'm not sure. But it does appear that a 45W 6.6k primary OPT should work for a Class A amp running with each tube Ip = 90mA. Hasn't anyone else puzzled this out before? Am I on the right track? |
![]() |
![]() |
#37 |
diyAudio Member
Join Date: May 2002
Location: Kiev, Ukraine
|
Well, all three of those have pretty similar primary DC resistance which is quite low (70-80 Ohm per side), so any of them will work.
The bigger ones will give you better bass extension due to higher primary inductance and also less distortion in the LF region, but for me even the 1620 seems quite reasonable, so it is more of a size/weight/cost question. |
![]() |
![]() |
#38 |
diyAudio Member
Join Date: Jun 2004
Location: Illinois
|
Rongon,
I have always interpreted the Hammond rating that is called “Max DC per Side” to mean the combined center tap current for two sides of PP, so I agree with your musings above. You may also consider Toroidy - the Baby Huey guys have had very good results with them. The TTG-EL34 transformer seems to fit your requirements, and at ~US82 per piece delivered to the US very competitive. TTG-EL34PP TOROIDY - Transformer: speaker | 50VA; O115x65mm; 0.008/54kHz; 200mA | TME - Electronic components Note - shipping by TME is $9.90 per package under 5kg, so you have to buy and ship two individual TTG-EL34 transformers, since these weigh slightly over 2.5 kg each. I bought several Toroidy transformers from them and TME has delivered them to me in central USA within 5 bussiness day in every case. Unfortunately TTG-EL34 is currently out of stock; if interested you may want to contact TME customer service - they have been very responsive when I contacted them. |
![]() |
![]() |
#39 |
diyAudio Member
|
Interesting info there. Thanks.
I have a query in to Edcor about current ratings and primary DCR of some of their models. Waiting for the reply. That TOROIDY OPT looks interesting alright. $82/each delivered is very competitive. My only concern is that toroids may require careful balancing of quiescent Ip of the output tubes. This makes things complicated. I suppose I could match the output tubes quiescent Ip. They are cheap enough. But they will drift with age, and I don't want to get into elaborate biasing schemes to keep the toroids from saturating on current imbalance. Have you found that to be an issue, and if so, how did you deal with that? I was looking at the Edcor CXPP25-7.6K. There's no current rating or info on primary DCR, but it weighs 4.5 lbs (2 kg) each. What's interesting is that they can be ordered with a single 4 ohm secondary. This allows better high frequency response (reduced leakage inductance, interwinding capacitances). Since the speakers I'll be using are either 4 ohm rated or smaller ported bookshelf speakers, I'd rather work with a 4 ohm secondary than an 8 ohm. (They're also available with a single 6 ohm secondary.) The price for those would be about $160 for the pair, shipped. If the 20-watt rated Hammond 1620 would work, perhaps the Edcor CXPP25 would also work. Generally speaking, I look for clarity and smooth mids and highs rather than trying to get the deepest bass response. I might even put a HPF at the amp's input to restrict low end response, hoping that might reduce intermodulation distortion. My speakers don't go below 40Hz and I don't think the amps should try to go lower than that. Amplifier LF response of -3dB at 15Hz would probably be fine for my purposes. The Hammond transformers are too expensive for what they are, but it was good to use them as examples for thought experiments. -- |
![]() |
![]() |
#40 | |
diyAudio Member
Join Date: Jun 2004
Location: Illinois
|
Quote:
|
|
![]() |
![]() |
Thread Tools | |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
groups buy TDA1541AS2 chips, money back gurantee | tube-lover | Group Buys | 222 | 24th October 2020 05:52 PM |
"Solomon" Solder Station - Worth the money or stick with what I've got? | Formula350 | Everything Else | 2 | 2nd February 2011 12:42 AM |
I require the best portable CD player money can buy. | mhouston | Digital Source | 7 | 9th February 2006 07:22 PM |
New To Site? | Need Help? |