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Forte 1 Kinda Bright on 8 ohm tap.


Mark51

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I just assembled a SE 6V6 tube amp with Edcor output transformers with an 8 ohm output.  I chose 8 ohms because that's what the specified impedance was in the early literature for the Forte 1.  But now I see that it says 4 ohms on the back of the speaker at the terminals.  I never noticed that before.  And the Forte's are kinda bright and bass shy on the 8 ohm tap.  Edcor only lets you choose one output impedance which kinda stinks.  Do I have any solution other than to order new transformers?  

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22 hours ago, OO1 said:

Nominal impedance of a Forte 1 is  8 Ohms  , you should be all set . 

https://assets.klipsch.com/product-specsheets/forte-brochure.pdf

 

 

On 5/21/2023 at 3:11 PM, Mark51 said:

 I see that it says 4 ohms on the back of the speaker at the terminals.  I never noticed that before. 

 

 The very first version of the forte 1 with the round input cup had a 4 ohm rating, at least that what was printed on them and I'm not sure if there were any changes to the crossovers when they changed the rating and input cup. 

 

 As far as the output taps I'm not sure it would hurt the transformers if you tried the 4 ohm taps since speakers sometimes have pretty wide impedance swings and their rating is just an average? Maybe @henry4841 or @tube fanatic could chime in? 

 

 

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Edcor OPT's are bought with just one setting for speaker impedance unlike the ones I am currently buying, Hammond, which have many options one can use. In general terms any of the multiple taps can be used on any speaker, it just increases power with a lower setting with slightly more distortion and lowers power out with the higher taps. Whether the OP will notice a difference changing OPT's is debatable but IMHO subtle at best. One can change the impedance of the speaker by adding some resistance in series or parallel but it is not something I would do permanently but it is something one may want to play with.  

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4 minutes ago, henry4841 said:

Edcor OPT's are bought with just one setting for speaker impedance unlike the ones I am currently buying, Hammond, which have many options one can use.

 

Thanks for the clarification I was assuming there was multiple taps for some reason I guess that explains why the Carver VTA-20 and Crimson 275 only have single outputs as they both use Edcor transformers my VTA-180's have  2, 4 and 8 ohm positive outputs and a single negative. 

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On 5/21/2023 at 6:11 PM, Mark51 said:

I just assembled a SE 6V6 tube amp with Edcor output transformers with an 8 ohm output.  I chose 8 ohms because that's what the specified impedance was in the early literature for the Forte 1.  But now I see that it says 4 ohms on the back of the speaker at the terminals.  I never noticed that before.  And the Forte's are kinda bright and bass shy on the 8 ohm tap.  Edcor only lets you choose one output impedance which kinda stinks.  Do I have any solution other than to order new transformers?  


Is this a SEP, or is the 6V6 triode strapped?  If you post the schematic it should be easy to come up with a solution.

 

Maynard

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Guys henry4841 was right.  It wasn't the output transformers.  It was some errors in the parts and circuit board which I purchased from Ali Express.  I know what your saying now - Well no wonder knucklehead look where it came from!

 

Well I wanted to try a tube amp after many years of transistors and this looked like an easy way of doing it.  I really don't enjoy building as much as I did but I like the $ I save and the end result.  I recently build a First Watt F5 which is a terrific transistor amp and that's got me back into building again.  

 

Look at the attached schematic.  Some guys on DIY Audio Forum helped me.  The grid stopper resistor in the first stage is in the wrong place and the 33mf cap following the 5Z4 tube is the wrong value.  It should be 4 mfd for that tube.  Once I corrected those errors and did some tube rolling the amp sounded totally different and I am impressed, after never hearing a tube amp into my Forte's.

 

An error that I probably made was buying the wrong output transformers.  I bought Edcor CXSE5-5K - 5W, 5K Ohms single ended tube output, which appears to be for guitar amps, when I should have purchased a XSE10-5K - 10W, 5K Ohms single ended tube output, which is more of a hifi amp transformer and has a UL tap.  I actually have them on order now.  But I must say the ones in there now don't sound too bad.  Not even sure I will hear a difference. 

 

I also plan to add a switch so I can go from UL to triode mode.   I've also attached a photo of the finished product so far.

 

Looking forward to more of your comments.  

 

 

208B2EC6-A1D2-439A-B729-95D93092E818_1_105_c.jpeg

A6409066-B36B-41F9-8633-27BFBD3B4D4B_1_201_a.heic

Edited by Mark51
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I do not see anything wrong where the grid stopper is at or the first cap on the schematic. The first filter cap being 33UF is right, nothing wrong there. You do not want the first a with a higher value than 50uf with a rectifier tube or the Voltage surge on startup would be too much. Most use a 1K ohm resistor as a grid stopper. Modern designers are removing the grid stopper and the 470K resistor as well thinking any stray electrons on the grid will find there way back to ground through the volume pot. You only need the grid stopper resistor if one has oscillation, squeal. If you want to play some remove those two resistors and put in wire on board and give it a listen. Less is best in my world. 

 

I've built a couple of those Chinese kits with some issues but after solving they sound really good. Iron appeared to be really nice as well. 

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Henry I was told the grid stopper should be between the grid leak resistor and the grid, not where it is on the schematic.  Well since they can be removed I suppose it doesn't matter a lot.  The data chart for the 5Z4 tube calls for a 4 mfd cap.  

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11 hours ago, Mark51 said:

Henry I was told the grid stopper should be between the grid leak resistor and the grid, not where it is on the schematic.  Well since they can be removed I suppose it doesn't matter a lot.  The data chart for the 5Z4 tube calls for a 4 mfd cap.  

I did not answer entirely correct on replacing those to resistors with wires. You cannot do that with the 470K resistor. You can just take it out and the stray electrons on the grid will find ground through the volume pot. You can just solder a wire across the grid resistor that can be easily removed to see if you really need that resistor. The designers I like try and take out all unnecessary parts they can. I do not see how it makes much difference where you put the grid resistor. There is no current on either resistor. 

 

The schematic does not show what rectifier tube is used and what I said was in general terms. If the data sheet for the specific tube used for the 5Z4 says 4mf that is what you need to use. But from what I see on the RCA data sheet for the 5Z4 it calls for less than 40uf so the designer of the schematic seems to know what he is talking about using a 35uf cap. https://frank.pocnet.net/sheets/049/5/5Z4.pdf.   But if your amp is quite with just 4uf of filtering for the first cap you are good to go. I just have not seen such a low value used before but the 5Z4 is an old tube and back then caps were expensive so they relied more on a choke for filtering. One has to be careful on what to believe on internet sites. Data sheets usually are what you depend on when working with active components and not what someone says on a forum. Not all advice is accurate or good. One has to learn who to trust on forums before believing all that is said. That includes me as well. 🙂

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More questions about this amp, which sounds really good. 

 

I have a bunch of those 5654 tubes and some EL84's so would like to make another se amp using them.  However, looking at this one, the resistance values around the 5654 seem awfully high.  Looks like a total of 2K for the cathode resistors when typically it seems around 470 ohms is normal.  Measured voltage at the cathode is 3 volts, which I suppose is about right.  But why so much resistance?  The supply voltage to the plates of the 5654's is 225 volts with 20 volts dropped by the 82K plate resistor.  Using ohms law that means the plate current is less than 1ma.  This makes no sense to me.  

6AK5 Triode - China Amp.png

6AK5 Average Plate Charactoristics.gif

6AK5 - RCA Tube Manual .png

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What is the voltage drop across both 1K cathode resistors? That and what is the accurate resistance from ground to cathode of tube of the two resistors. With both those figures using ohms law you can determine the current. Knowing the current and the voltage at the anode will give you the wattage of the tube to determine how you are biasing the tube. The plate dissipation of the tube is 1.7 watts and 80 percent of 1.7 watts is the target value most  designers shoot for biasing a tube. 

 

 

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I get only .0045W on that tube that is rated for 1.7W. Like you I think you could lower that values of those resistors. Om my calculations those resistors could be 250ohms each with a 1.25W on the tubes. https://ohmslawcalculator.com/ohms-law-calculator  Check it your self with the value of the desired W of 1.25 and a total of 500 ohms. Like you I do not understand why such high value resistors. 

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I lowered the value of one of them, the biasing resistor, to 500 ohms and I believe it gave the amp a more pleasing tone.  That moved the tube into a more linear mode according to the graph.  Perhaps the 1K resistor that connects to ground forms a voltage divider with the NFB resistor, setting the NFB level and operates independently of the 1k bias resistor it is in series with.  Sound plausible?               

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I gave that some thought as well, the nfb having some bearing but figures do not lie and the current you measured going through the two resistors is going to be constant no matter. If you use the distortion analyzer, Artar, or something similar you will have a better view of the changes you made. I assume no hotspot on the plates after changes, so you are safe for sure. Not sure how you decided value of resistance, through calculations or just data sheet but I used the calculator and put in the desired dissipation of the tube, 1.25W and the plate voltage and came up with the resistance needed. The data sheet does suggest 470 ohms so calculations were spot on. Running the 1.7W tubes at 1.25W is a conservative rating. I was hoping Maynard, our tube go to guy on this forum, would put his 2c's in this thread. I value his opinion. 

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What I'm going to do is just short out one of those two, 1K resistors - the one connected directly to the cathode.  That leaves a 1K biasing resistor and the NFB connected to the cathode directly.  I'll see how it sounds and that should get some more current flowing. 

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Turns out some of the problems I was having with the sound of this amp through my Forte 1 speakers was not caused by the tubes, the biasing of the tubes or anything else like that, but the very crappy Chinese coupling capacitors.  

 

I was in a local surplus electronics store yesterday and they had some Nichicon plastic film capacitors.  (see attached). So I got them just to see if it made any difference and it completely changed the sound of this amp.  Now I'm seeing why tubes are so good on Klipsch Heritage designs.  

 

Getting in a new Edcor output transformer next week so I will see if there is any further improvement.  

 

I'm seeing the problem with these cheap Chinese tube amps.  You might have a solid design but the quality of the parts may be terrible.  

Coupling Caps.pdf

Edited by Mark51
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